Any plastic skull that has good detail on the inside will make a decent mold. As RCIAG mentioned, the dollar tree is an option for smaller skulls. Here's a video actually using them to make concrete skulls:
You can check out larger skulls, but keeping them small will also keep the weight down. There are also many online options for purchasing skull molds. They vary in cost and also what they're made of. However, they may be what you're looking for. Just Google "Skull Molds" and you're going to find bunches.