Check out Ebay to find some cheap retail display mannequins if price is an issue, but just be aware that those types usually have idealized 'model' proportions, so that means longer thinner necks and unrealistic facial proportions. If your mask needs an accurate fit on a human head that may be problematic. you can also hit up any local charity or second hand shops, sometimes they have mannequins they are looking to dump/sell at cheap prices.
Sculpting on Polystyrene is no fun, its quite awkward and depending on the styrene quality, it can leave your expensive monsterclay with small poly beads in it when you finish the sculpt and remove it from the head form.
Hope that helps!