Wish I knew someplace good to buy single-stage paint economically

I'm painting antique cars (on which a base/clear would be incorrect), farm implements and shop equipment with single-stage urethane. I sure wish I knew of someplace that offered good prices on decent-quality paint. Probably this would be an online store rather than a bricks and mortar one(?). I'm thinking that maybe Summit's paint would be the best buy I am going to find, but then again, if I really wanted to be a cheapskate, why not Tractor Supply's paint (if I'm painting it black or red?). Any other suggestions would be appreciated. Thank you.