Unfortunatly, I don't think you'll find any good vendors for Prunus collected from the mountains in the U.S. We don't have those natives that they do in Europe. I've seen those tree Tony Tickle has and they are very beautiful! I have not seen any one in the U.S. collecting lots of flowering trees from the wild. However, it would be excited if somone proved me wrong and showed some awesome US flowering yamadori.
I'm currently on the hunt for a good Prunus Subcordata-Sierra Plum which would be Yamadori. These trees flower before the leaves. However, I also have a small plant that I plan to grow out and airlayer branches to create some long term projects. Have you looked up the native Prunus varieties that grow in the rocky mountains? There might be something that has old looking bark and flowers before the leaves in the rocky mountains, you should do some research. Keep us updated on your search with your blog!