Lowes has a sale on horse mats . $39 , but check first not all stores have the mats .
I've tried horse mat. I found that with a moderately powered airgun (say 35 FPE+) it works ok for a few shots, but multiple shots in the same area quickly tear through the rubber.
*Two great alternative for horse mats are: runners on top of concrete to soften walking/standing, and glued/bolted to the backside of a steel plate to reduce the sound.
IMO rubber mulch is the ultimate backstop.
I have seen Horse mats used as back stops at indoor archery rangesLowes has a sale on horse mats . $39 , but check first not all stores have the mats .