the vast majority of all monitors are 16:9 & 16:10 today. you are right that the 4:3 monitors seem to stop at 20.1 & 21 inch. the reason larger monitors (with notable exceptions) seem to follow widescreen or ultra widescreen is most likely due to the fact that pcs are used as media centers more and more.
i've never seen or heard about a large non-widescreen models outside of the consumer sector. such products should be available in the industrial market but i'm not sure you want to go that route.
if this is for a home display:
why not put 2x monitors in portrait view (for an overall squarish image)
use something like the dell 30" ultrasharp. it is widescreen, but theres enough resolution that you still have plenty of real estate no matter what you plan on doing.
ultimately: you need to say WHY you want a non-widescreen. without knowing the purpose it is hard to provide possible solutions .
I saw some 23.1 inch 1600x1200 monitors listed at prad.de, but most of them were from 2006 or older, so they might be difficult to find. A 4:3 resolution on a 30 inch 16:10 monitor has an effective diagonal of about 26 inches (I measured on mine). If that's not big enough, ssddx's suggestion of two monitors in portrait mode could give you an even bigger diagonal measurement, provided you can deal with the bezels in the middle.