Chippy,
I have gone through this exercise not so long ago. As you are most probably aware LHBS doesn't stock nitroger regs.
If you have a spare Harris co2 reg go to BOC & just buy a nitrogen adapter. Doesn't work with Micromatic regulators though. As the thread pitch is opposite.
I can't remember what i paid for the adapter at BOC Wagga.
Might be a cheap option if you have a spare reg lying around.
Cheers, Diffo