Look here http://sdrv.ms/1i4czGa for links to more catalogs than you'll want to read. Look up Apo-Nikkor and Apo-Ronar coverages, also Apo-Artars. Don't bother thinking about Apo-Saphirs, long ones are too uncommon. Don't forget Schneider's Fine Art XXLs, which will both more or less do for 12x20.
12x20 needs a lens with an image circle of at least (I rounded, don't quibble about a few lousy mm) 600 mm, 20x24 wants 800 mm. In Apo-Nikkors, a 760 is a bit short for 12x20, a 1200 or 1210 will do for 20x24. These aren't inexpensive.
Another example of the corrupting influence of found money, sorry, found film.