I've done a fair bit of online purchases, been pretty satisfied. I like to be sure of the manufacturer (who can go wrong with Hoffman challenge fabs??!).
Online shopping has been a really big help for finding treasures for my "secret angelee" - I can buy a couple of yards more for her, instead of paying postage getting a package to her!
A few of the merchants I've found through the FabShopHop.com site, some I've found on my travels, some from recommendations here :)