If you explore a lot you'll find a lot. A lot of the shacks have a few in them somewhere, and the RepConn Test Site (near Novac) had quite a few if you go through the whole thing, as I recall. Or if you have cash to blow, just buy them from the docs in certain places/towns. The one in Novac regens her inventory every day, last I knew.
And if you're using them up very fast in combat regardless, sharpen your combat skills or lower the difficulty.