Which back covers do you need? In the diagram below...#12 has multiple part nbrs for the left/right high & low beams....as well as the many headlight configurations (Xenon/Adaptive/non-Xenon/non-adaptive). If you input your VIN number at
www.realoem.com and go to the LIGHTING (main group)...you can click the diagram to find the 11 digit BMW part nbrs you may need.
Once you have the BMW 11 digit part nbrs...just type "BMW xx xx x xxx xxx" (where x= the part nbrs) into a google or bing search box...and you should get lots of results of vendors (BMW & online) that sell the parts you're looking for.
BTW, have you checked your local BMW dealerships parts dept? From what I see in the part database, the covers aren't that expensive (approx $13).