Check out the Fosters & Smith semi-automatic CO2 injection system, it is a good system for the price.
Read the reviews too.
They are VERY EASY to set up!
Mine is on the same timer as my lights for my 125 gallon.
You can get a 5 or 10 pound CO2 tank from welding supply companies & when you need a refill, you just exchange for about $16, after a 1 time deposit, which I paid years ago so I don't remember how much.
Look up CO2 gas or welding gasses, etc.
My 10 pound tank last around 3 or 4 months depending on how I set the CO2 flow rate.
Just make sure you measure the height where you intend to put the tank, my 10 pounder with regulator attached is 22 inches high.
Hope this helps.