i get alot of questions about what i paint on. i go to the home supply place and buy tileboard. its over in the paneling section of the wood dept. Its painted shiny white on one side and the other side is brown masonite. they use it for bathroom walls sometimes because its waterproof.
i cut it with a skill saw or table saw (depending on the size of the panel i want)
then i roll gesso on it with a foam roller three times and sand it lightly with 220 grit sandpaper for a super sweet smooth and absorbent panel. for $12 you get tons of panels for just a little work. give it a try. i think you'll like it.
here's an 18x24 getting rolled