You know Lew, I never thought about that. I buy coatings in "fives" so much I don't stop to think about smaller amounts. So I looked at Sherwin Williams "Rexthane" another good product, and it only comes in singles (one gallon) and five gallon buckets.
Rustoleum industrial/commercial makes a good deck paint I have used and the same thing... singles and fives. Took to Google and can see why there aren't many quart offerings. They are almost as expensive as the gallon sizes for similar products!
Maybe you could find the smaller amounts in an upscale hardware store with a good paint shop like an ACE or something similar. I have used the ACE products with no complaints from me or my clients, but can't vouch for its longevity.
Robert