From my understanding, any plastic chair will work as long as it fits safely(and doesnt "slip around" on the floor - I bet you could modify a white plastic lawn chair with those nonslip floor pads on each leg! or even some sort of old wooden (but treated wood or something) chair.

I don't see anything wrong with getting a shower chair just for the sake of enjoying a shower. Yeah they are made for someone disabled, but showers, if you enjoy them for them, are no different than any other form of pampering! You would buy yourself a foot spa if you liked that sort of thing, why not a shower chair for when you just want to relax in the shower?