Items and clothing. I dont play a caster, but Im pretty sure you can get armour that adds to your energy, plus items that can give you alot aswell. Also, there is a skill, umm, energy somthing,. that increases your energy by 3 whenever you spend a point in it.
As mentioned above, Elementalists have a primary skill called Energy Storage that increases the amount of energy stored. You were probably talking to Elementalists with 60+ energy as I believe they are the only class that can get that much. As a Necromancer, the only ways you can increase your maximum energy are via wands, staffs, off-hand items, and armor.