After completing wizards quests you can buy stackable 60 mana potions. As for warrior/rq they have no mana at all the only way to use scrolls that use mana is by unlocking ability that exchange health for mana at use
I figured I would update my Mana struggles. After completing the four Wizard Guild quests, I gained access to Icemist and can now buy Mana from Joanna. Just wish that those 4 outlying guilds sold Mana potions.