Google or eBay search on BMW 318i Power Antenna Hirschmann - E30. You might also find one in a salvage yard.
My 91 318i had it from day 1, and I ordered mine without a factory radio.
I got some practice doing an antenna mast replacement on the same antenna in a 1990 325iC, so this is what I remember:
A triangular brass piece with two 8mm hex head screws attaches the antenna to the car frame along the wheel well, while the black ground wire is secured with a 10mm hex head bolt.
You would also hope they ran the electrical wiring and connector (C219 -- two wire (red and black) back there.