Hi,
I have still not fitted my headlights as yet, as i wanted the electric levelling and headlamp wash/wipe to be in place first, headlamp wash/wipe is done just the electric levelling to do.
The ballasts are Bosch units and do have 2 different pt. no's 1 for the left 1 for the right, though electrically they are identical but have LH & RH brackets to fit the chassis.
If i was in your position i think i would buy 2nd hand (if they can be found) or failing that source new from Bosch.
I do seem to remember reading on the forum someone posting that they were the same as some other cars i think Audi was mentioned (just mentioning as this could be another source of 2nd hand parts), and other MB's I believe use them as well.
I personnally would hunt down some proper ballasts rather than buying some aftermarket kit.
The Bosch pt. No. for RH unit 1 307 329 039
LH unit 1 307 329 038
Hope this is of some help,