You can probably buy the parts on the web at either http://www.ackits.com
There is also an automotive bulletin board at http://www.aircondition.com
Lots of helpful advice there, and other parts suppliers as well. For a local source, look in your yellow pages. There should (?) be an entry under automotive air conditioning: parts and service, or something similar.
OK, clutch air gap. Look at figure 3 on the website I referenced - click it for a big picture. The clutch drive plate is pulled against the pulley (which is driven by the serpentine belt) by the magnetic force of the coil, when energized. When the compressor is not running, the clutch drive plate has some clearance ("air gap")between the pulley. This gap is adjusted with spacers. One is shown in the picture - it's the little round thing that looks like a washer, just below the "clutch drive plate removed" text. The air gap is supposed to be 0.50mm. It increases over time as the clutch material on the back of the clutch drive plate wears. You adjust it by using shims of different thicknesses. I suggest you buy a package of these shims as part of reconditioning your compressor. Any supplier that has the shaft seal will also stock the shims.
Last point. Are you certain your car has a 10P17? I really thought all the diesels were only equipped with the 10P15C. Better make sure. I think the shaft seal and shims are the same, but I'd guess the o-rings for the compressor case are different.