In the early levels for going balance (read this as 10-50 or so) alot of your best caster pieces will be cloth armor or the occational leather piece with a mix of Int and Stam. Int and Spirit combo is also good but the stamina will help in the short run as you will get little mana regen while you are fighting.
Until about level 55 on most of the pieces you will see with decent +spell damage will be cloth. At this point and further on into BC content it is easier to find.
Once you reach the BC content is where you should begin concentrating on finding gear with a mix of Int, Spirit, Stamina, mp5, +spell damage, and spell hit.
Targets for a balance druid for beginning to raid in Kara would be around 6500 health, 8000+ mana, +550 spell damage, 25% base crit (with moonkin aura), 12% bonus spell hit, and around 100-125 mp5 unbuffed. These numbers may be a little high but they should guarauntee that you do not go OOM in the middle of a boss fight.