I really don't want to try to make a yeast CO2 injector....but what i do have around are 3 20oz paintball CO2 tanks and a few 12's and 9's cant I just use those for my injeections? ill jus need all the valves and what not but cant that work?
I bought a kit, like this one for $106, which contains the regulator, a needle valve, a powerhead-driven reactor, a bubble counter with integrated check valve, some indicator-based means of monitoring the CO2 level and tubing. Actually, I invested $138 and bought the kit with solenoid, because I'm a bit paranoid with fish and CO2, plus it makes the CO2 last longer.
I think that desertgoldhound posted a link to a cheaper DIY setup that worked with an O2 regulator a few days ago.
I just started with CO2, and that's why I cannot tell you how long the CO2 will last in reality. I read something of about 5 weeks. Hard water makes it last a shorter while, whereas switching off at night will make it last longer (as mentioned for the solenoid).
The following is a pic of the reactor with the CO2 line in the tank: