If you need a less expensive alternative for a relatively temporary leased situation, it might be possible to find used conveyor belting for free or extremely cheap.
I know that it sometimes has a bad rap, as bare belting doesn’t have the same textured surface as mats designed for the purpose (although not slick, IME), and I’ve heard (but never seen) that some mats somewhere might have loose “threads” at the edges that could possibly be pulled at/nibbled by horses.
However, for many years I boarded multiple horses at various establishments that used secondhand conveyor belting in the stalls without issues – never saw/heard of any slipping horses, or saw/heard of horses chewing on the edges of the belt pieces, never heard any complaints from other boarders or the barn managers or owners. These were in partial self-care situations, where I was responsible for stripping my stalls, so I was up close and personal with the belting.
Unlike the 4 ft x 6 ft typical of stall mats, this stuff comes in long strips, and is narrower. So, the standard 12 ft x 12 stall, which would require six stall mats, would need three or four lengths of belting. Also, the belting, IME, is not quite as thick as stall mats, but much thicker than the thin rubber matting sold off rolls, and is very heavy duty. I had no trouble with the belting moving around in the stalls, either, so it didn’t have to be absolutely wall to wall to hold it in place (there could be several inches unmatted around the perimeter, for instance).
In my own barn, I have the usual stall mats, cut to fit, and I never even considered not having them. But, for use in temporary horse housing, the used conveyor belting – if available to you – might be a decent option. Of course, it must be bedded appropriately; during the time I boarded at barns with this product, we always used shavings for bedding, as pelleted bedding was not yet a thing. In my personal barn, on my “real” stall mats, I bed with pellets (great time saver).
TSC runs a sale on mats, every now and then. And I’ve purchased pre-owned stall mats off Craigslist very inexpensively (which I employ to cover the surface on which my gooseneck trailer is parked), so perhaps you might luck out there.