There is a kind of paint you can buy but if you basically want a board then it would be easier and cheaper to start with a piece of metal. The paint's great if you want to do something like an area of a wall that you then cover up with wallpaper and your kids can put up their posters with magnets without damaging the walls
Depends on your project as to whether they'd be suitable but cheap cookie sheets could be a good way to get decent sized pieces of flat metal with finished edges.
HTH!