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:
Charm of Mana - Hero recovers 1 extra spell point per day
Talisman of Mana - Hero recovers 2 extra spell points per day
Mystic Orb of Mana - Hero recovers 3 extra spell points per day
A secondary skill (Mysticism) also boosts daily mana regeneration rate.
The adventure map structure Magic Spring (found on rough terrain, the Stronghold's native ground) acts in the same way as the Mana Vortex building in a Dungeon town: it sets the hero's spell points to twice the maximum amount. A Magic Spring has two entrance points; each can be used once a week.