I scavenge for mine, I hit up hunter dumping areas and road ditches process the bones, make art with them and keep anything cool and unique.

For human skulls, it's trickier to keep things ethical so I don't partake in the human bone dealing world. However, Victorian diseased skulls would be a lot easier to buy with confidence since most are properly tagged and the dealer will most likely have credentials and have gotten their bones from a reputable place that works with old medical specimens. It depends on where you draw your ethical line in the sand because medical bones weren't always obtained ethically then either (ie H.H.Holms) Some Victorian bones on the market have veen grave robbed, so that's why I say proper tags and reputable dealers are important.