I’ve received a few queries from people looking for particular fabrics; they don’t have enough to complete a project or are otherwise looking for something very specific that they’re having trouble tracking down.
I don’t have evidence for this other than my LQS (local quilt shop), but I bet web stores tend to turn inventory over relatively quickly compared to the thousands of fabric/quilting brick-and-mortar shops around the world. So the latter stores are the place to look, especially for discontinued quilting fabrics. There are a handful of places on the web to post your fabric-finding queries. These sites are frequented by fabric store people who can hook you up!
Missing Fabrics: Post a free ad with an image of the fabric you’re looking for, if available. You can send in a swatch and the site owner will scan it for you for $1. She writes: “We do seem to have about a 70% ratio of finding what is lost and have even found fabric that is 25 years old before. You just never know. And it never hurts to try.”
Fabrics.net fabric search: for both retail and wholesale buyers.
Fabric Finder Forum from A.H. Mercantile: looks like most posts get a reply or two, so you might have luck here.
Quiltshops.com: Less for “lost fabrics” than for price comparison … this is a network of online quilt shops sharing the same software; just enter your search terms and you’ll get results from over 200 shops.