I have no experience with FSR's and unfortunately I can't find the link, but a while ago I saw a cheap FSR made of conductive foam. Most people here will know the black foam, it's usually used to stick chips in to prevent ruining them by electro static discharge during shipping.
The guy who wrote the article placed a metal plate on top and one under it. By squeezing it the resistance between the plates changes.
I probably would start researching cheap options like that since forces while hitting/kicking a punching bag can get quite high. I must say I really have no idea what's available commercially, but like you I'd be afraid to buy one and ruin it in no time.
So far I've seen two types, rather stiff and more elastic foam. I'd guess... the stiff foam won't have a long life, but can't tell you which would be best (if it works). I'd probably cover the sensor with a thick layer to prevent dents in the metal (making it unreliable)/not need a first aid kit.
It would of course be fair to add some arms/servos/sensors giving the punch bag a chance to hit back