Some of these fabrics are from my mom's and grandmother's stash (so they haven't been in stores for 20+ years), but I find the best scraps at fabric stores that are specifically for quilters. You can buy fat quarters in stacks which are smallish pieces of fabric. These can have some great detailed patterns on them. Also, the best quilting fabric store I've ever been to is
The Cotton Ball in Morro Bay, California! (I used to live there.) I miss that store very much. Maybe they do mail order?