We used sponge covered in tape for stools used by our grandsons. Not too pretty but saved the wall. Could you cover the arms with a pretty quilted fabric? I do not know on any "bumpers"you can buy.

Yep, had this happen whenever I had a party and drunk friends were on the bar stools. Solved by getting new, backless, stools. Sure, the drunk friends sometimes now fall off the stools but, my countertops are safe :)

I'm kinda joking but, the only solution really is getting new stools that cant hit.