Having spoken to a few people who do this for a living, there are issues such as testing for toxin contamination, unwanted additional organic material in the castings, etc. that come to play in selling the end products.
(No pun intended
There are a few in the area who sell the castings they make to local shops with little testing or none. It depends on the market and the regulations that affect your area and product. Perhaps a little discussion with producers in this area will help you, if they will take the time to talk with you. Do an internet search on the subject and see who you come up with around here.
Good luck & good hunting.