I've used cardboard and old carpet on my block target. You could also try and find some sort of foam at Home Depot or Lowe's. I'm sure it won't be the same but might extend the life. Expandable insulating foam? Never tried it but just a quick thought.
Oh a level 3 stab vest will work too. Kinda hard to find though. Arrow won't penetrate it fully. I've tried. (FP, Montec G5 & Toxic)
I stuffed the shot out insert with ripped up old shirts as tight as possible and wrapped the heck out of it with electrical tape. it has worked amazing.
My buddy found and bought some type of kit online. Stuff wasn't cheap, hard to use and generally it sucked as it would leave residue on the arrow afterwards. The old shirts idea is easy and works. Most bag targets are just rags stuffed into a bag, but it stops arrows.