We made our own. As others have indicated, getting the RJ-11 and RJ-12 isn't too bad - they're available at most places online (digikey, allied, mouser, newark, etc), and at Radioshack (our local store had both available). Add in a cheap $20 crimper, and you're set.
As for the wires... we got ours from our primary sponsor, Boston Scientific. As it turns out, one of the testing labs here was doing something with some wire terminals from another company. To get the terminals, they had to buy the whole cable (about 4 feet long per unit), but all they needed was the terminals on it. So they cut off the terminals, and were going to trash the rest of the cable. We snatched it up
It's got 4 wires running through a nice, flexible outer sleeve. The only downside was the color coding - two white and two black. That just means we had to use a multimeter to ensure we got them in the right order