Latest update: I got my regulator ($70 shipped) and I'm collecting the other parts I need. Meanwhile, I tried adding a second needle valve to my diy CO2 setup, but I couldn't get it to work any better then before. The other day, the plastic tubing popped off the needle valve and water started spraying onto the wall from the baking soda bottle. It was sticky too which I don't really understand. So I've completely abandoned the diy CO2 system. But it's going to take a while to build my regulator, so in the meantime I am dosing flourish excel. I bought a Burkett solenoid with buna seal on aquariumplants.com for $71 (shipped) with the plug already wired. The solenoid alone is about $50 + shipping. I could wire it myself, but it was only an extra $12 and I figure it was going to cost me a couple bucks to buy my own plug and it seemed worth it to save the additional shopping and labor. I also got used 5 pound CO2 tank from Amazon for about $55 shipped and called local places where they fill/exchange. Every local place that sells the empty tank charges about $80-85, so Amazon saved me about $30. It should cost me about $15 to get the tank filled. I still need a metering valve, a check valve, and all the fittings to put it together. I've been looking up metering valves with vernier handles. These tend to be the really fine adjusting ones that we need. I have found swagelok and Parker products available for $115 and up on Amazon and Sigma Aldrich. Once I make up my mind and get one, I'll be ready to put all the pieces together. A friend of mine is a chemical engineer and told me about a local place called F.P. Fensel. If I bring all my parts to them, they have all the fittings I'll need and I think they'll have the check valve as well. Some of these places are like warehouses and you have to know the part numbers of everything you need, but apparently this place is more like a specialized hardware store and they encourage you to bring your parts and let their knowledgeable staff help. My friend says the fittings are around $1 each. I expect the check valve will be around $100, but I guess we'll see. After it's all said and done my rig will probably have cost around $400. I'm not sure it's much cheaper than if I had just bought a custom rig from someone. It's certainly cheaper than the dual stage from GLA. And it has every feature I wanted:
-dual stage regulator with stainless diaphragm
-Burkett solenoid with buna seal
-Fine metering valve with vernier handle
-Check valve