Sgt Contrares (sp?) in the McCarran filed supply shack - he's had up to 1,000 rounds for sale at a time
Never seen him carry it. He does carry 1500 rounds of 12.7mm, and he's the only person to carry that.
This is what I do, with hand loader perk I buy all the hollow points they have at gun runners and the gun runners merchant under 188 trading post. They each have about 40 rounds per restock. Then, I just shoot those hollow points into the air/wall. With hand loader I seem to get back about 1/2 the shells from my fired rounds. Then I use primers from the .308 rounds I find everywhere and make .45-70 SWC ammo.
It's expensive and a hassle, but worth it with the cowboy perk to get SWC ammo for my sequoia and brush gun.