I am the proud new owner of a beautiful 15-91. I am wondering what I can use for a base for this machine? It runs like a champ and walked through several layers if denim at once. The feed dogs drop so I am interested in obtaining a walking foot or a darning foot for this machine.Where do I find these? It has several feet but none are these two that I can tell. Of course I have no Idea what any of the strange looking feet are for but I will be finding out by research.