I don't know if it is any use for you, but here in Finland one can still get them from
http://www.motonet.fi/ listing says:
- UMPIO (Whole headlamp unit) VOLVO 300-SARJA
300s 82--91 H4-lyhty vasen. . . . . . . . . . . . . . . . . 50-1857 174.00 ???
300s 82--91 H4-lyhty oikea. . . . . . . . . . . . . . . . . 50-1858 174.00 ???
UMPION LASI (Lens) VOLVO 300-SARJA
300-sarja 76--81 H4+vilkku vasen lasi. . . . . . . . . . . 48-4648 15.00 ???
300-sarja 76--81 H4+vilkku oikea lasi. . . . . . . . . . . 48-4649 15.00 ???
300-sarja 82--91 H4 vasen lasi alkuperäinen. . . . . . . . 48-4650 0.00 ???
300-sarja 82--91 H4 oikea lasi alkuperäinen. . . . . . . . 48-4651 34.08 ???
300-sarja 82--91 H4 vasen lasi ei-E-merkitty!. . . . . . . 50-1547 0.00 ???
300-sarja 82--91 H4 oikea lasi ei-E-merkitty!. . . . . . . 50-1548 33.90 ???
76--81: Pyöreä heijastin ja 82--91: Suorakaideheijastin
HEIJASTIN (reflector) VOLVO 300-SARJA
300s 82--91 H4 vasen heijastin. . . . . . . . . . . . . . . 50-1859 88.00 ???
300s 82--91 H4 oikea heijastin. . . . . . . . . . . . . . . 50-1860 88.00 ???
All prices are in euros, 0.00 means not available anymore. Less than year ago there was Mk1 reflectors and headlamps available and those were very cheap, but now only lenses, which are quite cheap too, maybe I should get few tomorrow.
You can get full list from
http://www.motonet.fi/?id=4 choose Volvo from box, then choose 300-sarja and you get list, google translate helps reading finnish
I think that I could arrange some limited part shipments as they don't deliver outside finland, but it is not going to be quick method because of my job that keeps me away from home.
Oh yes, if you see any gray when looking your headlights, then they could improve if replaced

Volvo 360GL -88 -under restoration-
Volvo 343DL vario -81 -running-

Volvo 240 Diesel -83 -undecided-
Citroen ZX Dturbo -97 -daily-

