Fabco are cheap and reasonable.
The Parkers on ebay are good(I use these and the next suggestion)
And the Ideal valves are perhaps the best.
Nupro/Swageloks are good also.
I'd not use a drop checker, IME, these have caused more issues and assumptions than they have resolved.
pH (and KH) likely is a better method, at least in relative terms and lag times.
KH can be off and has it's own errors, but it will NEVER over estimate the CO2 content. It can only UNDER estimate it.
So you can start with the pH/KH table, then adjust based on your plants and livestock slowly and progressively till they are all thriving. Done correctly, this works about as well as any method.