I live in an older house and want to install a tub surround - the back wall height is only 48" as the wall slopes. Can I purchase an acrylic tub surround and cut it down to 48" on all sides?

Yes, they are just sheet material. Be careful and make sure to put the saw blade in backward so it cuts going forward if you cut from the finish side. Otherwise, the acrylic will chip. You can easily cut this with a hand saw too.