Your tank spec is ok, so you shouldn't be running out of mana. Divine Plea will give you mana back, and becuase you have 2 points in Guarded By The Light, it will be refreshed each time you hit an enemy. Basically you should be popping it just before you go into combat.

It does have a 1 minute cool down however - so if the combat finishes mid way through the cooldown, you can either jump onto the next mob (if you can reach it within 15 secs - the duration of Divine Plea) or just wait until the cool down finishes before pulling your next mob.

Bottom line: Divine Plea should never drop off while in combat.

For additional mana you can also try judging Wisdom instead of Light, or subbing in a mana spring totem instead of a healing stream totem.

I would recommend changing the glyphs on your Pally however:
- drop the Holy Light glyph, you're tank should NEVER be healing (you can't dodge, parry or block while casting any spell thats not instant).
- drop the Judgement glyph - Prot paladin judgements don't do enough damage to make the Judgement glyph worthwhile.
- pick up Glyph of Divine Plea - 3% reduction in damage while Divine Plea is active makes it one of the best tanking glyphs
- replace Hammer of Justice glyph with Hammer of the Righteous glyph - having HoR hitting one extra is very handy for doing group pulls (i.e. most heroic trash).
-pick up either Seal of Vengeance or Seal of Command Glyphs - if you're not expertise capped, the 10 expertise gain from SoV will improve DPS since it lowers dodge/parry of mobs (benefits HotR and white melee damage). Alternatively the SoComm glyph gives back a nice chunk of mana with each judgement - if you're having mana problems this might be a safer bet.

Note about Seals -
Use Seal of Command on trash pulls - the cleave damage helps a lot here. Use Seal of Vengeance on bosses and any single target where combat will last longer than 20 secs. Once SoV reaches 5 stacks, it becomes the best DPS seal.

Hope this helps...

Cheers,
S