Try the Gun Parts Corp. in West Hurley, New York (914-679-2417). It's a big company that specializes in parts for old firearms. The catalog I have on hand is Edition No. 21, received a couple of years ago. It lists both five-round ($26) and 10-round ($28) magazines for the Winchester Model 1907, which has to be your model, as it's the only rifle Winchester ever made in .351.