I've actually had some amazingly good luck on Ebay. I was looking for some really obscure fabric and I was pretty sure I was either going to have to jerry-rig it myself or costum order it, but I found a guy selling tons of it on Ebay.
W00t.
The thing is, some of them try to trick you into thinking it's really cheap when it's not. Always check how much they're shipping it for and check all the details.
The fabric I bought, while it was an excellent find, came smelling a bit like smoke (something I failed to see mentioned on the ebay page) and I had to air it out for a couple days so it didn't bother my asthma. So if that kind of thing bothers you, be sure to ask. SEllers are usually helpful and friendly, but you can always check a sellers rating just to make sure.