Don't know if all the slaves are the same. If not, it might help if you post what bike you're looking to rebuild the slave on. On my Monster, there's a flat sided o-ring and oil seal that goes on the puck, a dust boot and an o-ring for the clutch rod. All in all, I think it's less than $20 from Ducati.
I have a complete factory original off my MTS with only @600 miles on it. Willing to sell it (best offer). Give me a day or two to insure i can still find it after my recent move to to TN. Pm if your interested.
DukeDesmo is right on. Evoluzione has a $20 piston that has two rubber seals on it. It will fit the OEM slave bore. Contact them for year/model application. They offer a complete replacement slave unit as well which many folks have used. If you search here and on the Yahoo ST site, you can review a few threads discussing the piston-only solution. Two or three owners have reported many many miles using the piston replacement solution.