Caster level enhancements are very powerful and equally rare. There are some spells that will do it, but none that I know with a duration long enough for crafting. The ever popular Orange Ioun Stone is the only item I know that isn't a short-term boost.
The Artificer class (ECS) has an effective +2 CL for meeting requirements, but not for actual effects. (ie, make a scroll of Fireball at 3rd level, but it only does 3d6 damage.) This is the only thing I can think of that sounds like what you're looking for. Creating an item of similar power, or one that actually added CL to the item's effectiveness, would be very pricey. Since I'm always suspicious of bypassing balance limitations I'd say a minor artifact or wonderous location. Not anything that can be gotten easily, in other words. Although a wonderous location like that would make a great goal for a story arc.