I search on www.quiltshops.com www.ebay.com www.etsy.com
quilt shops dot com features 200 quilt shops and usually somebody has something! One time on etsy I lucked out and found an "Our Lady of Guadalupe" panel...the cotton one with roses and I gave it to a friend after she swooned when she saw it! When I went to get another one there wasn't one to be found anywhere. There are other styles but not the one I bought from etsy!
Also when I search I use for example, the brand name (Robert Kaufman) the designer (Mary Englebright) the pictures on it (sun, moon, stars, dogs, elephants) style (plaid, homespun, solid, flannel, quilting, 100% cotton and so on).
I can sometimes locate a fat quarter or a small amount! Sometimes a quilt store will still have plenty.
The manufacturers of fabric really have an advantage because they do a run and then retire it and move on to new styles. It's so painful when you need it to finish a project or just want more of something you LOVE!