Failing all of the above, you can always loot some NCR cash off dead troopers and rangers. Kill them yourself if you don't care about the NCR infamy, or tag along with them on patrol and let the Legionaries or others they come across do your dirty work.
For that matter you could probably pickpocket NCR troops for it if your Sneak skill is good enough. Be prepared to make a lot of reloads if you're trying this at some NCR stronghold like Camp McCarran, though.
Your best bet to get large amounts of it, though, is to use the casino trick. Buy chips with caps, then turn them in for NCR cash. You'll take an exchange rate hit, though.