I know that some powers have the Reliable Keyword. Is there an item that prevents me from expending a power on a miss? I've read my share of books, but it is just too many books.
Not really, they would be much too powerful.
There are some limited items however, like the Pearl of Power from the Mordenkainen's Magnificent Emporium:
Trigger: You miss all targets with an encounter attack power of level 3 or lower.
Effect: You do not expend the use of the power.
But this is a daily power of a 7th level item.
What you can buy is more accuracy, which is actually better.
Even if you do not expend the power, you wasted your action. Still having that daily after 3 rounds of consecutive missing is not that great. Definitely not worth any investment.
- Eagle Eye Goggles gives you a bonus to Ranged Basic Attacks
- Laurel Circlet gives you a bonus to Illusion attacks
- Headband of Intellect gives you a bonus to Psychic attacks. Even better with a Githyanki Silver Weapon, as it can make all your attacks psychic
- An Accurate implement, together with the Superior Implement Trainig feat gives you a bonus to every implement attack
- Frost Weapon with the feats Wintertouched and Lasting Frost gives you almost permanent Combat Advantage, and a damage bonus as well