Went through this myself recently. Ended up getting a wheel from Germany and it was not cheap. Someone did a modification where they put a 4-lug monolever reardrive on a G/S. They took a GS paralever rear wheel (4 lug and spoked) and machined some metal off on the inside and popped it on.
Problem is, paralever rear wheels seem to be just as rare as g/s monolever wheels. I hope that BMW is hearing the increasingly numberous and loud cries for these rear hubs and does something about it.
BobFV1 to Putts: "Congrats on the bike. Also, congrats on having a dog with a perfectly heart-shaped asshole."
1991 R100 GSPD
FIRST TIME RIFF RAFF - Go Jonah!