Go to small tack shops and you can get them new for really cheap, or used for cheap. If you're in Maryland like me, go to Stablemates or TheGreyPony 🙂
I find mine at local yard sales! I just look for bootcut jeans in my size and I find them all over!!
Just jeans to ride in? I go to plados closet if you have one by you.. they are a high end consignment store so you can get miss me jeans for like 10 bucks!