Finding CRTs is difficult. Unlike most other parts in the set CRTs were normally rebuilt instead of replaced (the last rebuilder has been gone a few years now) so there are not large stocks of replacements around. The reason a Zenith tube is in a Motorola is that the original Moto CRT died and was replaced with a Zenith branded one.....CRT's of most given CRT type ID numbers (especially the common 10", 17", 21", etc. types) were often used in sets by various (sometimes all) makers.....Most CRTs are not specific to a single set or brand (though there are exceptions to that).
All that said if it is a 21" CRT (type# should start in 21 ie. 21ZP4) you are lucky. 21" sets are the most common monochrome TV, and the tubes are relatively common still.
You first need to find the type number of your CRT (likely on the tube layout chart inside the set or stamped on the CRT) then check this sites' CRTs for sale page (
http://www.earlytelevision.org/ ), Bob Galanter's site (he is user ohohyodafarted, and his site is linked in his signature), and if those fail post a want ad in the classifieds.
To be blunt 1950's 21" monochrome console sets are the most common and least valuable tube TVs around, and if it did not have any sentimental value to you (and you just wanted to restore a 50's TV) most here would advise you to scrap it and find a similar set in better shape. Those sets WITH a good CRT can cost almost as much to restore as they are worth after the resto, and the CRT replacement will likely set you back 1-3 times the set's value.