Totally agree with oracle, as a track car no problem but as street car too much effort, the savings on weight or gain on performance are so marginal... besides try to get a manual steering is another big adventure, I remember that it was an option for the RS on some years, and converting yours to manual is more complicated than the rebuild, you have to open the rack and drill some kind piston inside to allow some free moving.
Another source is pelican, ****** auto and another internet sales related to porsche, they sale rebuild racks with warranty and you can send the core later for some refund, and finally if you search ebay there is a lot of rebuilders offering units like this
http://cgi.ebay.com/ebaymotors/Porsche-911-993-Power-Steering-Rack-and-Pinion-Unit_W0QQcmdZViewItemQQ_trksidZp3286Q2ec0Q2em14QQh ashZitem2ea745d9daQQitemZ200374868442QQptZMotorsQ5 fCarQ5fTruckQ5fPartsQ5fAccessories