I am currently working on a sew and flip children's quilt, and the focus fabric is a Snoopy design from a scrub found at a garage sale. I have not seen that print anywhere else, and it is working perfectly for my quilt.
I think the best deal I have gotten at a garage sale in the line of clothing for quilts was a square dance outfit for $1. The skirt was sewn in gathered tiers each about 7 or 8 inches wide. The print was cowboy like Alexander Henry, and the colors alternated by tier. I got at least 2 children's quilts from that just using 6" squares.
I have a garage sale sophisticated cowboy print large ladies' shirt in the closet waiting for inspiration and the right colors to match. It looks like something from the 50's, and there is chartreuse in the print. The scenes are kind of desert and landscape, not so much horse and rider.
Anyway, I don't shop specifically for clothes for quilts, but I keep my eyes open, because of the unusual things that can be found.
Happy hunting!
Dayle