Each spell casting class in 2ed have a spells per day chart. There are you maximum number of spell per day you can cast, then you need to rely on magic items. This is the base system standard.
Only 2 rare exceptions to this rule exist in 2.0 ed the Sha'ir and the Shaman.
The Sha'ir has a elemental based familiar, called a Gen, that goes and steals the requested spell. The time the Gen takes is based on spell level and what class it comes from. Low-level spells are found quicker than high level spells. wizard spells are found faster that cleric/druid/other spells. The Sha'ir then can cast the spell 3 times, if he has the material components, before he has to ask the Gen to find it again. If the Gen steals a spell from a divine source, the god or other agent may notice and come looking for payment from the Sha'ir. The DM has control about whom the Gen steals the spells from.
The Shaman is from the Complete Shaman Handbook. It was made as a late "complete XXX" kit splat book that gave you a new base class. The Shaman worships spirits whom in turn give him a spell. Each spirit requires the shaman maintain a space within a shrine. More powerful spirits will require their own shrine but the grant higher level spells. Each shrine requires constant maintain from the shaman, so they are bound to a area or risk loosing spells by scorned spirits. Each spirit grants the shaman one spell, that spell does not change from day-to-day. After every time an spell is cast the shaman makes a wisdom check (roll under his Wisdom score on a d20) to see if he can cast it a second time that day. If they can cast it a second time they make a second wisdom check at +1. The difficulty keeps increasing until they are unable to cast that spell again that day.
The 2.5 ed books has some alternate systems for spell casting. Read them carefully, some were very limited with big draw back, like the Alienist systems. One was magic was not meant to be known by mortals and a wizard could go insane with each and every spell he learned. The other Alienist system you had a one free set of spells to memorize from you sponsor per level, you had to negotiate be able to memorize more spells. there was a spell point system and a recharge system. you really need to read them carefully before implement any of them, as they will alter the flavor of you game. They can be found in the Player Options books.
