I've seen wells that fully replenish the mana of a hero that visits, but since that is just a map feature, I can't always rely on it to be there. How else can I restore the mana of my caster heroes?
You can also restore mana by spending 1 turn in a castle/town that you own that has a mage guild built in it. It will restore the entire mana bar.
There are items:
A secondary skill (Mysticism) also boosts daily mana regeneration rate.