If you want a second hand door, do a search (google or yellow pages) for 'autoverwertung', which is the
germany term for car dismantling. Infact, here are the
Google hits for 'autoverwertung muenchen'. Then you have to find somewhere to get it fitted, I guess a car bodywork place. Sorry, dont have any specific recommendations for places for buying the door or getting it fitted. Alternarively, you could try to find a bodywork place, and tell them you want to keep it as cheap as possible, and ask if they could source you a second hand door from a dismantlers. They'd maybe have the contacts already to be able to phone round and see if they can find one.