They are not as plentiful as Predictas, but they do seem to be among the more common vintage TVs. Perhaps people kept them around because of the different look, or because they were expensive. When Grandpa spent a lot on something, whether or not it was any good, people tended to keep it.
<begin rant>
This is 100% hearsay, since I have never owned one, but what I have read over the years about other people's experiences in restoring Zenith portholes has made me lose interest in buying one.
They seem to be prone to eating flybacks. Some use a metal CRT, with the attendant concerns about shock hazard and premature failure. Although they do seem to work after careful restoration, I don't recall anyone crowing about the wonderful build quality, or the performance, for that matter..
The porthole screen makes a distinctive look, but I consider it a marketing gimmick. Owing to the rectangular nature of a TV image, you have to choose between showing the full image with lots of blank space on the edges of the circle, or blowing up the image to fill the entire circular screen and losing a lot of picture information on the corners. Neither of those choices appeals to me. Most manufacturers simply used a mask to let you watch the complete image without staring at the dumb unlit edges of the round CRT.
<end of rant>
So, that's my anti-Zenith porthole rant. If someone left a free one on my porch, I might take it, but I have passed up opportunities to buy them and will continue to do so. I know there are people out there who have restored Zenith portholes and are glad they did so. Perhaps one of them will chime in to explain why I'm just being crotchety.
If you feel that you must have one porthole, there are more interesting choices out there, such as a Stromberg-Carlson TC-10. Here's a nice example:
http://www.myvintagetv.com/tc10.htm
Raytheon also made some cute porthole tabletops:
http://www.tvhistory.tv/1946-49-RAYTHEON-BELMONT.htm
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html