Picked up one of the rather nice Victor SR310 CO2 regulators. As discovered by the Suicidal Tendencies team (archived).
Not cheap at around £170 from Leengate Valves in the UK, but they are near-perfect for robots:
In the US, you used to be able to get them from BOC. The Part Number was 0905-0163.
See the specs, with a picture of them there as they come, with input and output pressure dials. The body is 62mm dia, 130mm long.
I removed both dials - blocked off the input (high pressure) with a 1/4 blanking plug. The output dial is connected to a T piece with an output connector of a size that I didn't recognise, so I ditched the T piece and dial and a standard 1/4 connector screws straight in (black elbow at far end in pic above).
Discovered that the US CO2 connector is slightly smaller than UK version - you can screw a UK nut onto a US tank, but ya cant get the US nut on this reg onto a UK CO2 tank/extinguisher. Fortunately, unlike some regs, the inlet pipe unscrews quite easily, so I just replaced the nut with a UK CO2 nut (the large brass lump at the bottom of the pic above) - you can get these from welding supplies shops.
I machined off one of the fins on this one, to make it easier to get at the old-style extinguisher valve that we used on KHz. We have now moved to screw-down valves.