General concensus seems to be with Enaly. They can be bought on E-bay. They have a 200 mg unit and a 500 mg air unit. Both are reliable and live up to the warranty. Beyond that you'll have to replace plates.I personally don't believe their outputs are nearly as much as they say they are.
Others seem to like Aranizer, but if I were you I'd check the former posts on this subject. I wouldn't buy one again. There's way better units for the money. Customer service seems to be a concern. Some like Air Ozone.I can't comment on them because I have no experience with them.
A great little unit I found is on my website. It's the model 308 by AquaSun Ozone. It gives neg ions as well as ozone. Don't be fooled by the seemingly low ozone output specs on the unit. It cleaned up a 10X10 room that several cats had used as their litter box in just twelve hours. I don't have more than about 6 months experience with it so won't comment on it's longevity.If you want to look , the website URL is in my profile.
Hope this helps