Try some of the Talon ammo advertised in SGN and other pubs.
It's made from salvaged military components reloaded to spec. It's fairly accurate and economical.
Aimsurplus had a deal on Cavim in links, they are out now but said they are negotiating a constant supply of new, reasonably priced stuff from a European manufacturer, (Hungarian or Czech). Try them.
416Rigby - Talon has worked fine for me. Last I looked, http://www.classicarms.org had it for $79.95 / 400 rounds. It comes packed in a steel .50 cal ammo can. http://www.cole-distributing.com/specials.html also has a couple of different types. Best of all, they have in stock the Real Deal - genuine Lake City M2 ball manufactured in the 50's. I bought one 'spam can' - now I wish I had bought two! Hmmm.... maybe I will.
Cliff (lean mean link posting machine, this evening).