I always go to Sherwin williams for my paint. They have different qualities in different price ranges, but everything they sell is good stuff.
When I painted my vert the last time, I only used about half a gallon. It actually comes out to quite a bit more after you mix in the reducer/hardener. This was also for a basecoat/clearcoat, not single stage.
I always buy a gallon just to be on the safe side. Nothing worse than painting a car and running out half way through
If you go to Sherwin Williams, you can also give them a paint code from just about any car and they'll mix it up.