There are two ways to upgrade. The first is to find a broker and search the market for adept and master level ones spells that drop off mobs (alternativly, you can hope, pray, and dance naked on one foot whilst holding a dead chicken asking your favourite diety to have the spell you want drop ;)).
The other is to have the appropriate scholar (for fighters you need an alchemist) make you an upgrade at the app 3 or 4 level (common harvest) or adept 3 level (rare/impossible to get harvest. Particulary for level 20-30 spells).
Edited, Tue Sep 6 18:26:13 2005 by verawyr