I buy fabric on ebay all the time. Usually I'll start by searching for a specific fabric, perhaps "Lilac fabric" or "Lilacs fabric", or by the name of a fabric I've seen on another site, like "Northcott Stonehenge". I may search by color, such as "emerald green fabric". And then I narrow it to "Quilting" fabric. This gets rid of the non-cotton fabric (usually). If you set your search results screen to display 200 results per page, you can quickly scroll through and see at a glance if it's what you want.
Then there are several large fabric sellers and quilt shops that I've used before and bookmark, and occasionally I'll scroll through their stores to see what's new.