Mountain High sell them direct. I've bought stuff from them, to the UK.
Mind you, their mask (I've got one of them too) with the integral mike makes you sound like you have been sniffing something stronger than avgas
It does work but the mike isn't great.
With a mask, the o2 flow rate is very high unless you go for a demand regulator, and most demand regs in GA don't work with a mask. The MH O2D2 one does - $400 or so. It is the best demand reg though - I did some airborne tests for the PPL/IR mag and it was way ahead of the rest. It has some issues e.g. it fails SHUT when the power goes, and the batteries don't last that long.
But why a mask? Cannulas work fine to 18-20k although one has to breathe pretty consciously anytime above about 16-17k. The regs (mask above 18k) apply to installed systems only, not portable ones.