I don't know how your skulls should look like, just the face or whole skull (2 piece mold). We use silicon rubber for molds from the company Smooth-On (google it), but it's quite expensive. Other solution is to oil up the skull you want to reproduce, push the green stuff on and wait for it to dry. Demold, and oil up the negative and push the GS in. That way you have a perfect face of the skull.
If you realy need a lot of skulls, maybe we could make them for you, as a commision. If you're interested, let us know how many skulls do you need and how would you want them to look.