I usually buy all my model paint at Home Depot, Hobby Lobby or Jo-Ann Fabrics (they have it for like dirt cheap some days, its insane)
But, when I'm doing paintings of stuff, I buy it at this specialty store around me (I don't suggest doing this unless you're really good and passionate about it, the stuff I use is about 20$ per tube that's about 4 inches tall and 1 inch in diameter)