They still exist, I have a GAF and Kodak projector and a couple viewers.
I doubt they are manufactured anymore, but the used market must be available on eBay and such. I remember working with multi-projector setups back in the 1970s.
Probably the most interesting to see though was a 3-D dual 3.25 square projector that had been repaired and was used at a 3-D photography convention in Miami about ten years ago. It was big and finicky but gave nice bright 3-D. Originally it had been used by a major paving company to impress officials with a road-show travelogue that highlighted the changes in highways, especially through scenic areas. What made me realize that the era was over was when the presentation ended and the presenter repeated it with a digital projector. The digitized images had been color and gamma corrected, spotted, and looked fantastic - far better than the original material.