Many of Charlies greenhouse kits are made by B & C greenhouse which if you google can find their website. They're made in Canada. I happen to have one in the beginning stages-I'm starting the foundation. You can order them direct and save money, I believe. I've got the Premium one that I got from Craig's list for a great savings. Now to put it up!!!
My current g.h. was a kit from Conley. I absolutely love it. The two kits and manufacturers are very different. Conley focuses on quality and it's the strongest kit greenhouse on the market(that I've seen). Not cheap either. And you need to have a close relationship with a contractor to help you get it up. The directions are poorly written and leave much to the imagination. The B & C greenhouses have excellent directions, you can even order a video. The kit is extremely easy to assemble and doesn't take a professional. It's not nearly as sturdy, but is less expensive than the Conley. There are other kits on the market, but I would recommend you look into permits, county regulations etc. You'd be surprised to learn that there are only a handful of kit manufacturers that will supply stamped, engineered plans. Your county may not need that, but mine did and I had to stick with a kit that offered plans. You don't want to spend several thousand dollars on a kit and then find out your county will make you tear it down.