the fork seals on the R1100GS are the same as many small 125, 250 cc hondas , yamahas, y suzuki´s, and thus easy to find in SA. Javier in Dakar Motos produced a list when i was there last year and i was mildly surprised to see just how common the seal was. added bonus the honda part was only 60% of teh bmw part.
I am pretty sure that the 1150 is the same.
I don´t have the list of part numbers anymore as they are in a notebook back in the uk. But you could try emailing Javier, and probe his expansive mind, or visit one of the bike shops in Rio Gallagos or Punta Areanas. They may need to see it, but i reckon the odds are pretty high.
Or if stuck clean them then, trim the spring by a mm to tighten the seal.