@alterimagery: Thank you so much for your exhaustive reply! You posted exactly the information I was looking for in those links, and your description of the mechanics of film backs has me realizing that I will indeed use your suggestion in just using the roller-mechanism in my design. They do seem to be quite expensive to get on their own, so I think I'll go the route suggested by the previous two users - just get a cheap camera, and source just that part from it. This approach is even better, because just as rthomas' wife, I'd hate to go slasher-movie on a camera just for a percentage of its components

Thanks again to everyone, I'll make sure to post the end results on the forum...