You should not have mana problems as a protection paladin unless you are not fighting enough mobs at the same time, or you are so over geared you are not taking damage. I think its called Spiritual attunement that gives you back 10% mana for each Heal. So when you get healed for 1000hp, you get 100mp as well. Next is Blessing of Sanctuary. All blocks, dodges, parrys give back like 3% of total mana as well as a 2% damage reduction if I remember right. Also you should have Divine Plea talented so it refreshes on all melee swings. You also have blessing and judgement of wisdom if you need it as well.

Only time my paladin has mana issues in an instance is when im overgeared/leveled and I dont get mana back because I dont need heals.