Assuming your car. (like my 2002 example), came with a Motorola V50 with BMW firmware then thats what you need. If you don't get one with BMW firmware it will drain your battery when car ignition is off.BMW firmware phonbes have BMW logo on screen. ......................Disregard this advice at your cars peril.
I havent used the following site but appears legitimate - maybe do some research if concerned but seemingly £69 buys a working unit. More on this
clicky
Alternatively
retrobrick are 100% legitimate & been around a while............might even be associated to the 1st site I linked............I dont know though.
Alternatively keep an eye on ebay item 330515957401