There's one on sale on SAU (for sale section) I believe. As for other places, maybe wreckers, private sales or from Japan.
As for "it has everything". To be honest with you, I think the MFD is overrate lol, if I have a choice now, I wouldn't have bought one, would rather spend that money elsewhere. But I guess the MFD would raise my resale price if I was to sell my car.
As for wiring. You will need to splice into your ECU harness, triple gauage harness, speedo cluster harness. There's probably about 10-12 wires you need to locate and splice into.
But I do see your project being a more difficult job. Because with stock GTT's, they come with the triple gauage cluster, which is hooked up to the boost, and oil temp. Which a NA GT wouldn't have (the triple gauage & harness). That would mean that you'd have to locate those things yourself and splice into it. Because if I remember right, the MFD splices into 3-4 wires in the triple gauage harness.
I honestly reckon you shouldn't do all that to a NA R34. If you are on your P's and restricted to a NA, then you should just keep the NA until you are off your P's, then sell it and buy a GTT. Because the price difference between the GT and GTT is barely anything. And that way, you wouldn't have to muck around with upgrading all the brakes and stuff. It will also be a cheaper way to go.