My first fabric came from men's shirts from Goodwill. Specifically wanted plaids, 100 % Cotton. Also picked up Aloha shirts. Large, XLarge shirts have well over a yard of fabric, have been prewashed at $2.50 per shirt. Washed them and ripped them apart while watching TV. Frequently found some that were brand new. This is a great source.
WARNING: Be careful it can become addictive. Just two days ago I found 5 more for a scrappy quilt.