I've bought 3/4" 4x6 stall mats from several sources including Tractor Supply and all seem to be made of recycled tires. I like to buy a pallet of 25 at a time and use them widely around the farm.
A stall mat has to be hard and tough or a 1200 lb horse would destroy it. I tried one over concrete in the shop. The material are much too hard for mats to provide useful cushioning. At least for my feet.
The best I've use are much spongier and more cushiony mats I bought when on sale at WoodCraft.
BTW, here is one hint that helped me many years ago when I had to stand on concrete at a job. I took a different pair of shoes and changed shoes at least once during the shift. Changing the pressure points gave my feet a "lift."