Welcome to the board, Saint!
There's a parts forum on this board :
http://forums.pelicanparts.com/r1100s-bikes-parts-sale/
Also, a new parts source has begun here:
http://www.beemerboneyard.com/
Maybe surf on
www.craigslist.org
I think one of the BMW owner's clubs has a lot of classifieds, others should know about it. You might have to buy the pair, but you could try to sell the extra one...