Unfortunately, yes, it is only dark sages that give literacy. The black druid, monsters like ogre magus, the archmage (AFAIK), great wyrms, etc. do not in fact grant this skill.
If you haven't already, you should still poke your head down to the level(s) below DT. A lot of people do find that the monster they're looking for tends to appear in these levels. If not, one will probably turn up sooner or later in high kings or something. You could hunt dark sages in the big room, although it could take awhile. Make sure food preservation is over 90 to maximize corpse drops.
Hoping to win with every class, doomed. Archer, Barbarian, Bard, Beastfighter, Druid, Elementalist, Farmer, Fighter, Monk, and ULE Priest down.