going to get into CO2

For a tank that size I would definitely recommend a pressurized CO2 system. You should try to get at least a 10 pound tank. I'm sure you get away with a 5, but it would need to be refilled more often which is a pain.

You'll need a regulator to control the flow down to a low level. Then a needle valve to finely control the flow. Many people use a solenoid, so the CO2 can be plugged into a timer and shut off when the lights are out. You can buy these as all-in-one combos.

You'll also want an effective way to dissolve the CO2 into the water. I use the Hagen bubble ladder. It's not very efficient but does the job for me. It's very simple which is why I chose it.

several online sites

I would suggest that you investigate the traditional online aquatic supply stores, plus the online aquatic plant stores. I got my last regulator from PetSolutions, or ThatFishPlace, I forget which. I got the prior one from Aquabotanic, but shopped Aquatic-Store also. For an All-in-One, expect to pay $85-$90 for regulator, solenoid, bubble counter, needle valve combo. Add some tubing and diffuser or reactor, your choice of internal, external, cheap or expensive, powerhead driven, canister filter driven, or a diffuser which can be as simple as a micro airstone, a glass mister, or a Hagen ladder or similar thing. I've used the PlantGuild internal reactor ($50) and the Eheim diffuser ($15) and have been pleased with both.