You just need the bulb, or the entire assembly?
If the assembly is not damaged, but simply doesn't work, you might be able to repair it yourself. I did this a couple of weeks ago in my '83 240D. The front dome light never worked right, and would only occasionally turn on when I opened the door.
I pulled it out and took it apart (which required a little help, just another set of hands) - the contacts were filthy. not really corroded, but just nasty and fouled. I cleaned them up and put some electrical grease on them, and the light works fine.
'83 240D, 250k miles