There were a couple of kits available back when the cars were new, every once in a while they pop up on eBay or over on bfc. The main one is (was) the Century Turbo Kit, using components put together by BAE in Orange County CA. Don't bother trying to contact the company, they dissolved back in the mid '80s when the owner passed away. The kits ran RayJay turbos that are now very hard to find parts for anyway. Callaway also made a kit for the e21 M10, but they are even more rare to find. Best bet currently is to contact Top End Performance in LA CA. They sell an exhaust manifold to bolt up the turbo, then you can sort the piping locally.
Depending on how much you want to boost, you don't have to go to Megasquirt or anything like that. The CIS is capable of supporting about 9-10 psi with basically no modification. Some Volvo 240 Turbo parts (also CIS) make things a little bit easier, namely the vacuum canister for the distributor and the warm-up regulator (to add some more fuel under boost conditions).
What do I use one of my BMW's for BETWEEN Bimmerfests...
BMW CCA AutoX Class wins, 3
Second places....and counting!