Petpet paint brushes are not a real item in the game. Neopets has a wide variety of paint brushes for pets, but they don't have any specifically for petpets.
To change the look of your petpet, you'll need to use a Petpet Paint Brush. These come in many different colors and patterns, and you can find them in the Petpet Shop or by trading with other Neopians.