My experience leads me to believe that it's the pull that has worn out and not the zipper itself. I've seen Randy Logan cut a zipper pull off and I was amazed at the kind of wear that occurs to the inside of the pull. The teeth of the zipper are probably ok. The pull is what meshes the teeth together, and it seems the teeth don't wear nearly as fast as the pull.
If you don't have a local person who can do this, ship your bag with both parts of the zipper to Randy Logan in Knoxville at Logan Magic Shoecare. He keeps a variety of sizes of zipper pulls, and it doesn't seem to be that big a deal if you have the right replacement pull and know what to do. Cut the old pull off. Pry off the little stops at the end of the zipper. Slide the new pull on. Replace the stops. Simple. Right? Randy's replaced just about every zipper pull on my Aerostich and other gear as well.
