These cables are quite simple to build youself. I've built several. I use separate cables for recording and playback. Both are 15 pin d-sub to rca.
Apogee gives a pinout here:
Apogee Electronics: Products: Duet
For playback I used pins 5, 10, 14, and 15, for recording 1,6,11, and 14.
The d-sub connector is available at RadioShack (and many other places). The RadioShack model # is 276-1501 and the housing is # 276-1508.
Beware that the screws on the sides are useless and the housing will not fit the Duet unless it is filed down. One way to deal with this is to build the cables and just before sealing the housing, fill it with epoxy. Close the housing, attach the screws (the ones that hold the housing together -- don't use the side screws) and let the epoxy harden. After it has hardened, file flat the top and bottom of the front end of the housing so that it will fit the Duet. (Do not file before adding epoxy, since it will then be difficult to nicely position the connector in the housing.)
I imagine that any microphone wire will do nicely. I used Mogami.
Once done, these cables are a breeze to use, fit snugly just pushed on, and they are a lot easier to work with than the more massive Apogee breakout connector.