Headset Services will sell you a Gentex if you ask - although they seem to carry more Alpha stock than Gentex.
I ordered a Gentex HGU-55 from Flightsuits in the US a few years ago, and there was a month's worth of dicking about with export paperwork - not sure if Flightsuits will still export a complete helmet though.
The Alpha is arguably better made than the Gentex, but is quite a bit more bulky which can be an issue if you're restricted for headroom. Also, spares for the HGU-55 are a lot easier to come by at reasonable rates, whereas Alpha spares can be fairly eye-watering.
If you can solder and work a multi-meter, I'd consider sourcing a raw HGU-55 shell from Flightsuits, get your liners, earcups, speakers, leads and microphone from
OREGON AERO,Inc.- Oregon Aero designs and manufactures more than 500 products for aviation and other industries that prevent and eliminate pain, reduce noise, and improve impact protection., and put the thing together yourself. The one benefit of a factory custom build is having the edge-roll fitted to your face, but I'm sure that with a bit of effort you could have that done locally.
If you're in a high noise cockpit, consider some secondary passive noise reduction since both the Gentex and Alpha don't perform as well as a decent headset in this respect. I use the CEP system,
Communications & Ear Protection, Inc., and I know Headset Services will sell you custom-fit earplug speakers (but at a considerable premium).