They used ballistic gelatine dummies for a while, then decided that they weren't a good model of most injuries, so had an episode where they designed a new dummy by buying human bones and breaking them to see how much force they could take, then picked out a material with similar properties (which surprisingly ended up being wood), and from then on, built fake skeletons to put in the gelatine dummies.

IIRC, the bones were bought from a shop that just sold human bones. They'd ended up there nominally because their previous owners' wills had permitted it. E.g. lots of people want their skull to be used in productions of Hamlet, so drama groups often end up owning real skulls. I'm not sure whether Mythbusters was buying things from a general-use shelf or if they counted as medical science due to doing some kind of experiment - the US military counts even if they're just using corpses for target practice.