I have a custom built Co2 Regulator that has never been used except for testing, for those that don't know this regulator is built with the best components to give you accurate control without constant adjustment. with the needle valve I can get down to 3bpm without trying you may be able to get it slower but I honestly never had a need to slow it that much.
Burkert Brass Solenoid Valve Type 6011
Smith Medium Duty Co2 Regulator S series model 30-100-320
Swagelok Check Valve Model SS-2C2-TR-5 5PSI 1/8"
JbJ Brass Bubble Counter MA957
Swagelok needle valve SS-2MG4-MH
It has all the necessary fittings and is assembled. it can also be made more compact by moving a few fittings around I just wasn't concerned about spoace.
Price $160 shipped cost me $260.00 to build.