Human hair is probably easiest to source in quantity, given it is a waste product in hair dressers world wide. Other current uses are wigs, and cleaning oil spills.
In the Dutch public TV program "Keuringsdienst van Waarde" they had an episode about bread improvement ingredient (keratine) and traced it down to being sourced from human hair collected at hairdressers in China. Sounds disgusting of course, but many raw materials are, and after processing industrial grade pure keratine is the output.
