I don't think there is anything special about the BMW seat heaters; I know others who have replaced them (or installed new ones) with aftermarket heaters.
A few things to note: the Z3 heaters are one piece that folds between the seat and back. You can't replace one or the other. Also, disassembling and re-assembling the seats is a big job and even worse if you have sport seats. I recommend taking lots of pictures as you go since lots of pieces fit into other pieces but not before other pieces, etc. I also recommend replacing the hog rings with zip ties.