NECA is making an xenomorph skull available at some point, it looks pretty close to the one in PredatorII.
BUT...
It's going to be foam filled latex. I personally wouldn't waste the money on that material, it won't last forever...heck, it might not even last eight or nine years before it starts degrading. So it would end up a waste of money.
There was a guy here who went by the name Fantasy Creations, or Fantasy Designs...something like that, and he was making 1:1 sculpts of some of the alien skulls in the PII display, and he was going to mold and make copies to sell, but there hasn't been word on that in years.
The easiest piece to do is the human skull. Tons of human skulls available all over the place.