temple of essence

Temples of Essence stack. Get enough of them and max mana won't ever be a problem again.

Oh yes it will because it doesn't reload. You end up with lots of empty/unloaded essence points which you never have the time to fill up unless you spend 50 turns doing nothing. The only units that benefit from that are the children who also often reload mana end hit points thus becoming almost invincible uberunits.

There is a Tower that you can build which will grant +2 mana points to regen for each hero channeler.

I understand the Sov's offspring regenerate mana in tactical combat.

There are quite a few mana bricks/tablets that work like mana potions.

Are there any other ways to increase mana regen? 

I said MAX mana would never be a problem again, as in you can easily get to the point were pretty much all your imbued heroes can theoretically cast spells requiring 30 mana.

Actually having the mana to cast those spells is a different problem all together! The building which Gene1966 mentioned which increases mana regen from 1 to 3 is the only really helpful thing in this area that I know of.

Yeah, it is very easy to have 400+ max mana per channeler by the end of the game since you have to take droves of ai cities to find their leaders.  It wont matter once they change the entire magic system though so I wouldn't worry too much about it.