I think you need to have a budget,,, you can likely inject pressurised CO2 to 6 aquariums for $100-150 not including the CO2 tank (At least in the US)
But are you sure it is necessary?
I think the Ocean Aquariums thing you are referring to is known as passive CO2, you will not get the same levels of CO2 in the water as you will with active pressurized, but you basically have an empty bell in the aquarium, and fill it with CO2 gas periosically as needed,, the water will drain from that as time goes on and plants use it.
DIY methods, either Yeast or baking soda methods are cheaper to start, but a bunch of work and high ongoing expenses.
with pressurized, you don't run it when lights are out because plants will respire CO2 and it can lead to too high a CO2 concentration in the water/ using a pH controller bypasses this, but makes it more expensive still.
honestly, in the same position, I would skip CO2, and just choose appropriate plants and light levels and run from there.