have made large eggs out of plaster of paris, as well as ones *out of paper. don't see why you couldn't play with it to get a head shape. I use the balls kids play with for a mold.

use cloth mache instead of paper. It is more durable and will withstand the chronic hat on/hat off, moving around the shop or to/from swap meets, etc.

I don't know if there is a web site, but decades ago I bought a book called "Screamers" where he makes cloth mache monsters.

I buy sheets from thrift stoes or you can buy muslin from fabric shops. You can use a balloon or beach ball as a base, but I think you'd have better results with a foundation of chicken wire. You can shape it to what you want much easier than a balloon. I use watered down white glue to paste the cloth strips. I get a huge bottle from hardware stores. Assemble as you would paper mache.

The drawback is that the cloth holds more water than paper so takes longer to dry. You can keep your watered down glue fresh by covering with plastic wrap.

if you want to be able to make a bunch of heads of basically uniform shape and size and dont want to buy a ton of them with creepy faces or strange hair (i used to have some of those cosmetology heads and they are fun but.. kind of ..unsettling) i'd suggest finding one glass head.. you can sometimes find them at marshalls/tj maxx/aj wright/homegoods... or online...
then you just smear some petroleum jelly on it and paper mache it... when it's dry, cut on the sides of the neck, in front of the ears, and over the top of the head. pop the 2 pieces off, seal em up with more paper mache, maybe fill it up with something (put a beanbag in the bottom of the neck so it stands up and wont fly away in the wind at outdoor craft fairs) and paint it up to make it interesting.
then you can do as many as you want and leave the glass head at home ... if a head gets damaged you can just make a new one!
maybe spray them all with some of that cool stone-look spraypaint. my mom made a wicked creepy gargoyle head and used some of that and it looked so awesome.