In addition to
http://www.quiltshops.com/index.html which has already been mentioned, I use three other fabric search engines:
http://webstore.quiltropolis.net/
http://www.fabshophop.com/fabsearch.asp
http://search.fabricshop.net/cgi-bin/sitesearch.cgi
I also use Google images (looking through images is much easier than looking through websites). There's not much I haven't been able to find with this group of websites. :oops: