A few options.
Infinicar in the UK has at least two servotronic boxes from euro 3.8 cars, one of which is on ebay. Both should fit your car directly. You may have seen them mentioned on m5board, as one of their current parts cars belonged to a member over there. They want big money for that box, though. I am looking for the same thing to retrofit servotronic and a tighter ratio onto my 535 and my parts are coming from germany.
Another option is to buy the rebuild kit. The rebuild kit is just a bunch of seals, no hard parts. The part number is 32 43 1 162 736. I have no idea if the cut off date means anything, but...given that the pitman arm is the same across all E34, I would bet that the seal you need is in that kit.
Edit: oh, and another idea -- Will Zaraska (
www.willz.ca) has a canadian spec '93 M5 parts car that should have a servotronic box. Price will be up there.