All you need is a standard AC-DC adapter like you buy at Radio shack, with a barrel connector. The Triaxis is a bit of a pain because it is a fairly narrow socket, with an unusually thick center pin. If you try a few, you will find one that works though. Bring your Triaxis with you when you go shopping (trust me! Most don't fit, some do.) The output voltage (AC or DC) depends on what your foot controller requires, as does the current draw. Usually foot controllers specify 9V DC with a negative center terminal, but some accept 9V AC. They typically draw between 500 and 1000mA. Just get a 1000mA to be safe.
The other thing you will need is a suitable length 7 pin MIDI (DIN) cable. The two extra pins carry the phantom power to your foot controller. They can be harder to find but if you look around you will eventually find one. I've seen them on eBay.
The GCPro with accept a 9V DC too, make sure it's got 1000mA+. I have run my GC with a 9V DC adapter. Voodoo Labs themselves assured me it was safe. The first circuit in the GC is a bridge rectifier circuit which converts AC to DC. Still, it's more precise if you get an 9V AC supply.
