monster makers have a good one..
http://www.monstermakers.com/category/sculpting-armatures/
but if you are looking for on the cheap you can get a foam head for wigs at your local wig shop for less than 5 bucks and then make a stand for it using pvc and wood. only problem is the wig heads are not quite full size they tend to be a little small. or you could get some alienate and mold your own head and then pour plaster into the alienate mold and then you know it will fit you.