The headband is the way to go. It's the standard Int-boost item and has the lowest cost per point, for an ongoing effect. Other items, like the scarlet and blue ioun stone, are more expensive for little additional benefit; don't bother with them unless your headgear slot is already accounted for.
If you only need a short-term boost, you could use fox's cunning. For wizards it is basically free. A non-spellcaster can use it in potion form, 300 gp for 3 minutes. Unfortunately it doesn't help for stuff like bonus spells or skill points.
Note that all the above effects grant enhancement bonuses, and so they don't stack with each other. If you have a headband +2, and an ioun stone for +2, plus the spell for +4, your total bonus is still only +4.
To get an additional bonus that stacks, you'd need to wait until you can afford inherent bonuses. Those start at 27,500 gp for a +1, so you won't be seeing them for several levels yet.