The main high altitude flying course in the U.S. is Guidance Helicopters out of Arizona...
http://www.guidancehelicopters.com/index2.html
They are a Part 141 training center, which many folks feel is better than the standard Part 61 training center. I met the owner and two of their CFI's when I was at the RHC Safety Course, and they seem to be a good lot.
I don't have any specific information about the school; nevertheless, they seem to have an excellent success rate. Also, they are opening up a second location in Montana shortly.