I try to acquire the animal parts I use (bones, etc.) as respectfully and ethically as possible. No animals were killed for the use of their bones/etc in my art. Some were found dead, some killed for food, some naturally deceased, killed as pests, etc. I'm operating under a "better to use it than throw it away" theory, which I hope you all understand.
Similarly, all the old books, clocks, etc. that I use are damaged to the point of non-functional. I don't destroy anything valuable or useful, and try to only make things that improve the starting materials.
Almost all the sea shells I use are ones I personally found on the beach, although some were traded for, or purchased second hand.