A cheap CO2 regulator is avalible from welding shops for small MIG welders. Usually has a high presure gauge so you'll know when its just about empty. They usually have a 1/4" pipe thread inlet. Adapt that to the paintball remote hose with high presure fittings. Then the solenoid, a needle valve, a check valve, a bubble counter and then to your injection device of choice.
Thats how I made mine, minus the solenoid at the moment. The solenoid I had laying around wasn't rated for 100% duty cycle and over heated. My tanks still lasts about 2 months with my external reactor into my 75 gal bow front tank.