Good news and bad news
The good news is that I have baked a couple of Susan's sourdough using Wheat Montana Natural White All Purpose flour and find that it is much like KAF Bread Flour. In fact I get the same oven spring and the flavor is wonderful. I bought a 10lb bag because it was slightly cheaper that way - the price here on the island is about the same as KA. Because I have limited storage space I asked at the "bakery" in my grocery store for icing buckets and they called to say they had saved a couple for me, one 5 gallon and one 1 1/2 gallon.