A while back I bought a box of 100 Remington cases (head stamped R-P) for the 357 Maximum. No back order, no waiting. I ordered them from Midway USA. I checked with Midway, and they are back ordered, but expect it to be available on 3/27/2012. The cases are definitely available, you just have to keep looking around and be ready to pounce on it when you find it in stock.
The wait will be worth it, because you will love your Maxi, I promise !