Any store that carries ammo will stock .45 ACP, but many will not stock .45 Colt, which is the round you need. The image below shows the .45 Colt on the left (often called .45 Long Colt) and the .45 ACP on the right (often called .45 Auto). If you have only one round to look at, the most distinguishing characteristic is that the .45 Colt has a rim that's a little bigger around than the case. That's what lets you load it in a revolver without the round falling out through the cylinder.
Hope that helps.
"An armed society is a polite society." — Robert A. Heinlein
"After a shooting spree, they always want to take the guns away from the people who didn't do it. I sure as hell wouldn't want to live in a society where the only people allowed guns are the police and the military."
— William S. Burroughs