I've just finished dealing with a similar issue with my gearbox intermittently disengaging and the PRND lights flashing.
After several visits to mechanics and electricians I was told it's either the selector unit or the mechatronics.
In my case I took the mechatronics out and had it refurbished and my problem is fixed. That's not to say you have the same issue, but from what I was told the problem can be from both endss of the circuit.
Anyway regarding the selector unit, I could only find a new one from Germany for 400€ I think. Might be worth looking abroad if your set on that.
Another option I found was that apparently some people have fitted Mk6 units to their mkv, I only found one fortun thread about it in the usual, but maybe that's an option, If I remember correctly they had to change the cables with the mkv ones, but it looked relatively straight forwards.
Of course I have no first hand experience so that info might be off base. But might be worth checking into.