Gla sells up-aqua.com glass co2 diffuser. And the atomic diffusers creates a co2 mist that most say is unappealing.I have the ista one. It is cheap & does not work well. All the other ones on the page are cheap.
Green leaf aquariums look the best but if you go to their home page they are always out of stock. Sorry I am not much help. The CO2 diffuser is important & most diffusers are cheap & will not last long.
This. Alternatively, you could get an in-line CO2 reactor for a little bit more if you don't want to see the diffuser and tubing in the tank. The CO2 mist will be just as fine, but you may find that disconnecting the filter tubing and soaking the diffuser is a pain. You may want to invest in an additional diffuser so that you can always have one clean one to swap in while your other one is being cleaned (I recommend this for all atomic diffusers, not just in-line ones).get the super atomizer type.