I was going to suggest checking Beemer Boneyard, but a quick look indicates they don't have any suspension parts that old at the moment.
Also note that if you shorten the suspension, odds are you'll have to shorten the kickstand (not a terribly difficult job if you know a competent welder), and your centerstand may become nearly impossible to use. BMW made a lowering kit for the Hexhead GSs, consisting of shocks, side stand and center stand, but it only fit bikes made from late 2006.
Bottom line, you may be out of luck. If you were planning to trade up anyway, you might just want to do that rather than spending a lot of money.
BTW, 40K miles and 14 years is not particularly "old" for one of these bikes (mine is 13 years old with over 70K miles). There are lots of people still riding around and racking up serious miles on Oilheads (1100s and 1150s) with well over 100K on the clock.
Whizzy brakes, no FD drain plug, what's not to like?