Hi, Doug. Check the board for the Wood Mizer site or link to it from www.woodweb.com. Ask them for a list of sawmills in your area. Then go to the small local sawmills and tell them what you need. You should be able to find somebody to cut barn siding that will match what you have.
You will probably want to save the old siding. It is valuable to people who want to use the old weathered boards for paneling. You will have to cut out areas of rot in your old siding to make it saleable.