I was just about to say the same. There will be piles of Halloween tat being given away for pennies in a couple of weeks. You could use that or use it as a mould.
You often get punch bowls or trick or treat baskets in the shape of skulls too which might be usable.
Are they for outside, decorating the track, or inside? If outside, you could find a cheap skull bowl and fill it with concrete then cut away the plastic.
I have seen a cracking skull today in local shop about 75 cm tall 55 cm wide with leds for eyes asked where he got it woman thinks it was last year but from costco can take a pic if you like
I'm actually going to Costco tomorrow morning so will have a look in there!
In regards to the other comments that's also a good shout! They are for inside just random places.. I did envision a wall of skulls once upon a time but I'm not sure how feasible that is haha.