back in the day u used to be able to just walk through the supermarket and find caps on everything and just cop em.
unfortunately they've gotten a bit smarter...but if u still just walk around the supermarket or dollar store ull be suprised at the caps you'll find on stuff.
the easiest way is probably to order a pack from artcrimes, bombing science, or whatever. they're not very expensive so it really isn't a problem. or u could always just learn to paint with stock caps 