Spandexhouse is pretty good, but I hate ordering from them because I had to talk to about 5 different (possibly the same...?) people before making any progress. And it was long distance.
is another good one. They have pretty much the same selection as spandexhouse, they also mail out free samples, you talk to one person when you order, and you get your stuff INCREDIBLY fast. (Like, next day fast, sometimes)
As for instructions for sewing it... buy a stretch pattern (a cheap one) and follow it at least once before messing around. That's made me the spandex goddess I am today. ^^;