The city where I used to po-leese had a pawn shop that would sell you individual cartridges.
I saw a number of folks over the years come in and buy 5-6 rounds at a time.
It was handy for me too. I once held a gunsmithing FFL and I could buy a few cartridges to test fire a gun if it was a caliber I didn't have on hand-.25 ACP, .32 S&W, .32 Short and Long Colt, etc.