We brought some equestrian rubber matting (so expensive :?) from a company called Numat for our two stalls - cost us almost $3000 for two loose boxes. The original floor was old concrete in pretty average condition so we put down some self leveling concrete over the top first to make it more level. For the price of the matting we didn't want to cause additional wear from the uneven floor. I think the guy said you could put it down on a dirt floor if it was compact and level enough but we already had concrete in there. It hasn't had much use yet but will do over winter.
We bought our mats from the Canadian version of Tractor Supply. They're laid on top of a clay base with a light layer of base gravel on top of that (some people call it road gravel - mix of sand and smallish stones).