BMW has great support for older cars, if you don't mind waiting a little while. The parts department will need proof of ownership...expect a service fee along with the key blank & cutting charges. In the States they want to see the title, not a registration card.
When they're checking that info for you at the dealer, ask them if they can print out the original build sheet for your car. Some dealers will, some won't. The one tacked to the rear bench seat has most of the information but I like to have both. It'll tell you the build date of the car, the options installed at the factory (like diff ratio) & a bunch of other useful info.
You also might want to pick up a few cheap dealer-only parts like coolant bleed screws, plastic fasteners that have broken, etc.