I know here in Australia you can buy just about any rear shock unit (e.g. one to suit a GSX-R750) and send it to a suspension specialist who will re-build it to suit a Ducati. My mechanic has just had this done for his 750F1 (yes, he bought a used GSX-R shock), it came back with a shortened body and all new internals to suit the F1. The best thing about doing it this way is that all up it was half the price of a new Ohlins shock, as good as new.
On a side note, had my suspension 'fixed' on Friday and went out on a track day on Sunday, the bike is now much more confidence inspiring to ride with the suspension sorted. Re-built the forks with 8.5kg/mm springs and a gold valve kit, fitted an Ohlins steering damper and rear shock and a spent a bit of time setting it up. Overall the ride is firmer but not as jarring over bumps (much better dampening), doesn't dive under brakes as much or want to stand up while braking into a corner and now doesn't squat and wallow under acceleration.
All up a great improvement, should have done this mod first!
s4's rear shock will fit '02+ 2v'd monsters. '01 and earlier has a different shock length, no? the s4 has the single tie-rod for the rear suspension while the '01 and earlier monsters have the "hoop" that ties in to either side of the swingarm. i'd take some measurements before looking to buy.
that said, when i scored my ohlins, i got about $75 + shipping for my used sachs with 6k miles on it. (reference?)