I've sewn lingerie at various times over the years, and one of the most difficult things about lingerie sewing is finding the correct fabrics and especially the notions. Fabrics are much more available than notions, although some of the specialty fabrics are tough to find, too. I promised BeeBee I'd give her some elastic sources, and she suggested I put it on Stitcher's Guild so that everyone would be equally enabled, and now I've been asked by an anonymous commenter to post the information here as well.
First of all, I have a post here (linked on the sidebar) called Fabrics and Findings Source Links that gives links for many online fabric and notions sources, including lingerie sources. I've been keeping this post updated since August, 2005 as I find and try new sources. The date of the most recent editing will show at the bottom of that link list.
A little idea of what you might find at these sites is:
Elastics for lingerie are often going to be against your skin rather than encased, and what you want is the plush elastic, which has one side that's very soft. There is also strap elastic, for straps, which is stronger and less stretchy. You can find these plain or fancy, and the plush often has picots on one or both edges to give a pretty finish that peeks out. There are some with ruffles or lace on the edge(s) as well. I ordered some printed elastic too, which is very pretty. Narrow and wider stretch laces can be used in place of elastic for panties too. Styles seem to come and go, so if you see something that's unusual, you may want to snap it up as that may be all there is. Don't forget that most elastics and findings are easily dyeable.
Underwires and channeling (a pre-made casing for the underwire) used to be the sticking points if you wanted to sew bras. They just were almost impossible for the individual to find. Now they're more widely available, and all of the sites I've been looking at carry them. The sizing is now the sticking point. Some of the sites measure the length, some measure the widest point across and the depth. It's very difficult, but some sites have PDFs you can download with scans of the wires to compare to your bra or pattern. I noticed that one site, ELingerieA, sells sets of 6 pairs in different size ranges so you can pick the one closest to you and then compare a range of 6. This seems like a good idea if you're not at all sure. None of the sites have interchangeable sizing either, so that's unhandy as well.
I would also suggest that if you're even slightly unsure as to whether you're wearing the correct bra size, you check out the (long, but very worthwhile) thread on Stitcher's Guild called Bra Fitting--Expert Tips. Even if you think you're wearing the right size, you may be surprised and pleased by trying some of the tips Dotty gave us from her years as a professional fitter. I know I changed my outlook and bra size a lot after finally taking it all seriously and doing the work of finding the right size.
All of the sites seem to carry about the same categories, but they all have different products for the most part, so there's a wide variety available. Some of the sites offer kits for bras, which is handy to start with because you'll get everything (or almost) that you need to make a bra. For the kits, you will have to order underwires separately, and also powernet if you want to use it, and for larger sizes, it's really necessary.
There's a thread called Lingerie Findings and Notions on Stitcher's Guild about this very thing, as I said above, and I'm going to copy some of the info from that thread here, with the vendors in alphabetical order, except the vendors Zora noted on Stitcher's Guild are all at the end.
Bra-makers Supply is a Canadian company. They have all the supplies you'll need to make bras, undies, and other lingerie. They also have a blog with lots of info on how to do it all. They carry patterns, underwires, closures, very nice jewelry quality findings, elastics, fabrics, etc. There's a lot of help available here if you read through all of the posted information.
ELingerieA is a German site that has bra kits, underwires, channeling, all the findings for bras and panties including lots of lovely elastics. They also have different bra patterns than I've seen on other sites, with a much wider range of cup sizes. I still ended up tracing my pattern from a bra I have, but they do go up to an H cup in most band sizes. I found their pricing to be excellent, and their shipping on my first order was exemplary. They adjusted the shipping on my second order after I contacted them, and it was acceptable.
Fabric Depot has a low-tech website, and you need to call or mail in your order. They're in Texas. I suggest you call, because Susan is extremely knowledgable and is very happy to help you choose just which products you need from the huge inventory she carries. She has 9 different types and weights of powernet, for example, and it's nice to have her tell you which one she thinks would work best for your project. Elastic, underwires, hook and eye tape by the yard or by the piece, bra cups, closures, etc. She also has everything you need for skating/dance/swimwear/activewear including patterns from KwikSew, Jalie, Ezi-Sew and Elan. There's a catalog available to order, but she sent a free one with my first order. They also have what they call Valu-Packs, which is fabric, elastic, lace, lining, etc. by the pound or multiple yards with several colors/styles included. Could be very handy.
Lace Heaven has lots of elastics, and some other items. The prices are very good, and I think they are sort of a close-out place, with whatever happens to be going, rather than a regular stock. For example, I got some very nice double-sided plush elastic with Natori woven into it.
Lucy's Fabrics has some elastics and stretch laces.
Needle Nook Fabrics is Anne St. Clair's site. She designed the Bra Elite patterns, and has a full selection of lingerie fabrics and findings. You must call her to order, and she is worth talking to.
Pattern School is an Australian site which has lots of information on bra pattern drafting and explanations of the why's and how's of it, as well as letting you take "the lazy way" if you don't want to do it from scratch, by entering your measurements. It will spit out a pattern for you that is said to be a good start. I haven't tried that, but it looks like it would be worth investigating. The man who owns the site, Stuart Anderson, is a swimwear designer and does custom work besides.
Sew Sassy Fabrics is another good source and they carry underwires, channeling, elastics, fabrics, etc. as well. I've ordered from them in the past and been very satisfied.
Spitzen-Paradies is an interesting site for 'eye candy' and ideas. It's all in German, which is no problem for some of you, but not so good for others of us. The Inspiration and Gallery links are especially nice, but the whole site has lots of interesting photos of different lingerie items made up in different ways.
Bra Components is a new source (to me anyway) which will custom dye very small amounts of findings to match any Pantone color#. This would seem to be ideal when you need a very particular color match. They don't have a huge variety of styles of lace, etc., but when you're getting a custom color, that may be okay.
And here are the links from Zora, with her descriptions. Thanks, Zora, these are great links!
Kantjeboord is a legendary Dutch lingerie shop. They are specialized in kits complete with lace, fabric and notions and they ship international. Mind, the website is all in Dutch.
Merckwaerdigh is a dutch lingerie pattern maker who sells patterns and notions on ebay. She speaks English and she ships international.
Sewy is a German shop with patterns for lingerie and swimwear. They also sell notions and kits complete with pattern, fabric and notions. The website is all German but Bärbel is really nice and will certainly answer English emails. Sewy patterns have a reputation for well made instructions with lots of illustrations so if you understand German, give them a try. I think Elingeria sells Sewy patterns with English instructions, but I'm not sure.
Stickgarne-Martha is an Austrian seller on ebay and she has the cheapest lingerie notions I've ever seen and mostly good quality. She's a smoker though and sometimes what she sells smells of cigarettes. I solved that problem with the help of my washing machine but if you're allergic, you better don't buy there.
Wien2002 is an Austrian shop specialized in lace and lingerie notions. A lot of the stuff is sold at bargain prices but the quality is good. Again the site is all German but Angelika ships international and is very friendly so if you find something you like, don't hesitate to email her and ask.