Leroy - my wife has a Presto pressure cooker for canning. All replaceable parts for it are available in our local hardware store and the local farm stores. She also had a couple of pressure cookers that she inherited from her mother and grandmother that parts cannot be found for. Anywhere. We've given up on them.
Sometimes you can coax a ring to make it thru one last canning by coating it with cooking oil before closing the lid. Yours sounds like it's way past that though. Good luck.