Again on EBay..Cookie cutters.
Cost~ Less then $5.00 (Australian) all up including postage.
The product 'Das' comes to mind, though any (wet) plaster or similar product that you use in your scenery making endeavors will work which enables an imprint. Just add the facial details yourself with a lot of artistic license.
The third one is included for those that want to include a little ol' romance in their adventures.
Example: Adventurer 1. "I say ol' chap looks like another skull we have here"
Adventurer 2. "Yes indeed my fellow academic, so it does"
Adventurer 2. "Seems to have been formed useing a cookie cutter"
Adventurer 1. "POPPYCOCK" where in god's name did this tribe ever have the ability or technology to get their hands on such a thing?"