For BMW repairs out of the official dealers, I found Janoo Motors in Al-Qusais (near Sharjah border but in Dubai). He deals only with BMWs and has old and new ones and probably knows where to get you a cheap upgrade. Owners name is Ali.
A friend got his convertible hood changed from there at a good price.
For Junk yards, my experience is that you need to go around. There are a few in Rashidiya in Dubai, a few in Qusais and a few more in Sharjah.
I've got some stuff from Al-Khoory garage in Rashidiya (next to Bin Dhaher motors on Steel mill road). They have a junk-yard + a garage and are good at sourcing stuff from other junkyards if needed. Owners name is Wajid.
Tell them that autodealer.ae pointed you their way