Jeans are cheap at thrift stores, but not just any sewing machine will sew through several thicknesses of denim. A commercial walking foot type is the best for this. The newer home sewing machines will not be up to the task of sewing old jeans together with upholstery thread (regular thin cotton thread will not last long on a seat cover).
About 35 years ago, my brother got into upholstery and found an old Singer machine at a yard sale. It was a home type, but was very heavy duty. He got some big needles and some upholstery thread, and started doing car seats. The motor on the machine finally quit, so he put a 3/8 chuck B&D drill motor on it. This old machine would sew through your finger if it would fit under the foot! Years later, he acquired a commercial walking foot machine and used it for years and then sold it to a friend when he quit doing upholstery. A couple years ago, I bought the machine and still have it. I may break it out and try doing the used jean seat cover idea.