Whenever I look for ammo, I normally always check ammoman.com first.
At the latest gun shows I've been to, haven't seen much .50 ammo, and that which was available was $2 a pop or better.
Been meaning to pick up some of the Barrett ammo, just for the collectibility of it, but just haven't gotten off my butt to do it. It's supposed to be damned nice stuff to shoot.
As for a shooters bench, I bought one a while back that seems sturdy enough. Starting to show deterioration from being out in the elements for years, however. The Florida heat and humidity will get to just about anything after a while. Wish I could think of the name of the guy I got it from. Three quarter inch plyboard top on a steel square tubed frame. My .50s fit on it well and didn't slide off from recoil.
As for primers, been a long time since I bought any, and they were CCI that I got from Cabelas, I believe. I would check Wideners or Hi-Tech Ammo if I were looking for some now. If you are a member of the FCSA, they publish a source book every year that would probably come in handy to you.