I stumbled on to the Quilting Board while looking for sites to post fabric requests or sites to respond to.
These are the ones I've found so far:
Looking for Fabrics
Fabric Want Ads
Fabric Finder Forum this is on a menu that goes across the page near the top
Forum - requires a sign up to post
SewitsforSale@yahoogroups.com - requires a sign up
I have not posted a request to any of these sites, but I have responded to requests from the first seven sites.
I don't know how the last two work.
Hope this is helpful.