I made some wasteland bases a while ago, with skulls littered around. I found the best way to get lots of cheap skulls quickly is to press mould them, using plastic sprue (which might also be the best use for sprue ever).
To make the mould I just made an impression with a skull in a piece of Green Stuff. Then to make more skulls, I cut up some sprue into long straight shafts, and heated one end with a lighter (usually till it just catches fire), then pushed the hot end into the mould (with force), then just wait a few seconds for it to cool down. When you pull it out, you have a perfect (if slightly chard) little skull face, which you slice off, rinse and repeat.
Disclaimer: obviously, melting sprue can be dangerous. Plastic releases plumes of toxic black smoke when it catches fire, and will drip melting hot plastic around, which will cause horrible horrible burns. So this should only be attempted outside, by an adult, with suitable safety equipment.