Really depends but I'm not 100% sure if there's a massive market for it given how few of these fittings people typically use. The pressure doesn't factor into the legal bit, but what speed the BB comes out of the muzzle is. Some solenoid HPA systems (i.e. electro-pneumatic) will run at as low as 40-50psi (e.g. Wolverine, Polarstar etc.), but some fully pneumatic systems like a GBBR-to-HPA magazine tap or Daytona Gun will run much higher (at around 120psi). I've not known anyone to run a sustainable setup that exceeds around 130psi.
The actual bits one would want to buy are usually quite affordable (with some exceptions). I've never spent more than £3 on a standard fitting:
http://www.ebay.co.uk/itm/NPT-Pneumatic-Push-In-Fittings-for-Air-Water-Hose-Tube-ALL-SIZES-AVAILABLE-/272732115526?var=&hash=item3f8019d246:m:mKFd6UHvcTTXC_lC3sjtjLA
For the vast majority of people, they'll never use any push fittings because these days it's standard procedure to use an
IGL (integrated grip line) that directly plugs into the engine itself. For others, they might buy a single 6mm to male quick disconnect for around £6, but very, very rarely will anyone need to spend more than that on fittings for a gun because there's only one way the air gets in and then the rest is handled by the engine itself. There's no advanced plumbing or anything.
The exception would be the Mancraft M.A.S.S (basically an air stock) that uses about 3 such fittings, but again these come in at around £5 a piece and I believe two of them are included. Whilst they're metal fittings that would typically be more pricey, they would appear to be
cheaper through Mancraft than the metal fittings available in the UK. The only other example I can think of where someone might use a fitting is for a GBBR-to-HPA magazine tap, which is a 1/8NPT to male quick disconnect usually. Again, not more than £5 from eBay but you might be buying 3-4 at a time.
In short: Not sure if it's worth your time I'm afraid. HPA has been refined a lot in airsoft in the past 5 years and manufacturers really don't want buyers worrying about push fittings and the like, and in the case of some of the more complex systems like Daytona Gun there are off-the-shelf airsoft-specific solutions and bear in mind these are guns that cost at least £600 to build (usually around £800-1000 when all is said and done) so a few pounds ontop of that purchase just doesn't factor in