Originally Posted by
Tartan
If you are going to be machine quilting, you can keep an eye out for sheets on sale. Look for nice quality sheets with a large floral or design that you can outline stitch to practice FMQ designs.
Sheets can be very difficult to quilt (hence why a lot of quilters tell you NOT to provide them for backing). If I was just learning, I'm not sure I'd want to use them. You are a lot more likely to get skipped stitches.
I don't have a lot of experience with clothes either- but honestly, if the goal is quilting, not piecing, thrift fabric might make it harder to learn. The problem with sheets is usually that there threads are closer together, so old men's shirts probably don't have that problem. Don't use knit if you are learning.
Personally, I'd buy sale fabric at the store or online. Fabric.com often has some under $3/yd. If you just want to practice your quilting, make wholecloths. They use less fabric, and take less time, than something that is pieced.