I get my paint at Wal-Mart (in the automotive dept.) and AutoZone. AutoZone is about $3.50 USD and has a broad range of colors, Wal-Mart has fewer colors but the paint is cheaper at about $3 a can.
Wal-mart and Hobby Lobby both carry plasticote paint but I have only been able to find plasticote primer at Hobby Lobby.
HTH