Done a search on this site for garages in istanbul and found an address from a traveller called Ozhanu which was submitted 4 weeks ago.
He says theres a good mechanic with all sorts of parts for different bikes?
The garage is called pitstop motorbike shop and garage.
Address is Besyol Inonu Cad.Pakten Mutlu sanayi sitesi
Phone +90(0) 212 599 09 72 - 0212 624 12 90
Mobile +90 (0) 533 478 10 87 - 0537 2266176
Ozhanu has titled his thread as "A good garage in istanbul,Turkey" just in case you want to search this site for his message.
Also found an address for BMW in istanbul on the Bmw motorcycle owners of america site:
Mototal-Mototas A.S.BMW(sina and umit afyoneri)i think these are the owners?
phone +90/216 347 6171
Address:intas sitesi A
Email:email@example.com or www.mototal.com.tr
Hope these help!
Best of luck