I can't help with finding these in the aftermarket.... however, you do bring a good point. Your extended warranty covers 'major components only', which usually means engine and drive line. I would certianly press the issue of headlights being a 'necessary' and 'major' component because without them, you can only drive 10-12 hours in every 24 hour period. Also, the fogging of the reflector and lens could be blamed on improper curing or the epoxy... when the lights are on, the epoxy gasses and fogs the assembly.
Try sweet-talking your dealer first, you may be suprised on the action you receive once you plead your case NICELY about the lack of SAFETY in your SAFEST CAR ON THE ROAD!
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles