Last time I had a problem like this to solve I took all the bits to a hydraulics place:
They know screw fittings well; I told them what pressures I was working with and they sorted me out. Obviously one needs to be aware of potential oily residues (with items not designed for breathing systems), so you might consider cleaning any components thoroughly before use in a breathing situation. (Disclaimer: use of any item for a purpose it's not designed for is potentially hazardous, so don't do this unless you know how to do it safely!)
The other approach is to go and talk to one man band dive shops. Such establishments usually keep a box of old parts, sometimes useful to plunder.
Finally, there is still a residue of old equipment originating from John Buxton with the Northern Section, which may just have something of use. I'm far from certain this source will sort your problem, so not worth going out of your way for. But if you happen to be in the Dales, talk to Northern Section members about accessing this material.
Maybe something above will help.