I'm not sure about Clayton but the Tower Grove market is supposed to have grass fed beef, pork and lamb. Check out the vendor list.
BTW, what do you consider the best true farmer's market in the St. Louis area? By "true" I mean produce and such sold by the folks who grew/raised it.
FYI, I consider Soulard to be a wholesale market not a farmer's market.
Well, Soulard really isn't wholesale, but there are small stores that buy there, also wee restaurants, but there are lots of individuals. Clayton this week had Prarie Grass Farms' fabulous lamb, as well as someone selling beef. Remember, it's now in the parking lot west of Straub's.