Lockers, crates and metal boxes are the best places but random spawns.
NV seems to have far less then FO3, more so than most other misc items.
OT: I have been running around a OCD pack rat lately, with no specialized skills and have been picking up a whole bunch of wierd things I usually don't pick up. I know I've been picking up every single cherry bomb because, like you, I wanna make some bottlecap mines. I think, IIRC, that I find the majority of them in mailboxes. Cherry bombs in mailboxes. Bunch of damn pranking punks running around the wasteland. Next thing you know, there'll be flaming bags of brahmin excrement on everyone's doorsteps.
I've playing a trader too Gunny, I just collected 35+ radscorpion poison glands from giant ones all over the Mojave...
Decided that a trader would make a killing with exotic food stuff, 50 or so caps from each stinger turned into 2 caps per portion worth of food.
The PC got screwed by Ruby Nash, and now her barter skills dropped 50 points.