Yes those Designo (and quilted) seat skins are able to be purchased from your local dealer. You simply disassemble your existing seats, remove the stock skins and replace with new OEM leather skins. However, expect to pay an arm and a leg for them. A good resource to ask may possible be "Christheswede" on this board who converted his W221 S Class from a regular interior to Designo. He was able to source the parts second hand for a really good price and then swapped out his interior.
Another good resource is "MBenzNL" on this board who deals solely with OEM parts and retrofits.
Otherwise if you go the aftermarket route, Brabus may be a good way to go but also pricey.
Anything else, I consider to be not worthy of a car such as the S600. Other cheaper solutions/ custom shops won't have the right workmanship or quality (IMHO) that the car deserves.