The way to get it is to have Celia or Lede cast Ultima on Ramza as a Squire. The best place is at the gate of Limberry Castle. They won't always cast it by themselves, so you'll need to change your equipment setup and speed to persuade them to do it. The Ultima Learning FAQ by Archmage Zorlock has more information on getting Ultima.

EDIT: It helps to have your navigation messages turned on (in Options), because then a prompt will come up asking you if you want to learn Ultima. Say yes, of course.