My mom collects old jeans. As ours wear out or get holes, we give them to her. If they still have lots of life and are just too small, we usually donate them.
She also buys them when the thrift stores offer $0.99 jeans days. She looks for the older ones and larger ones. You can cut a lot of squares from size 52 jeans!
If she sees denim on clearance at the fabric store or as remnants, she buys that, too.
You could post a wanted ad on Freecycle asking for worn out jeans, holes okay. Lots of people just throw them in the trash.
Or, just ask around to everyone you know. You would be surprised how fast the jeans accumulate once people know you want them. :)