I love buying shirts and fabric at yard sales and thrift stores. Our local Goodwill has a policy that on Sunday one tag color is $1 an item. I have been collecting plaid shirts for a quilt. There is usually at least 1 and sometimes closer to 2 yards in every shirt.
If you're buying fabric at the thrift store that isn't marked, pull a couple of threads and burn them with a cigarette lighter - pure cotton, silk and wool will all burn to ash, while anything with polyester will burn to a small hard ball. DON'T pull more than a couple of inches of thread. That's all that's needed and you really don't want to set the store (or your fingers) on fire.