What cards give +1/+1 counters to all friendly creatures?
Ajani Goldmane's -1 ability will give counters to all your creatures and also give them vigilance until end of turn:


  −1: Put a +1/+1 counter on each creature you control. Those creatures gain vigilance until end of turn.


Ajani Steadfast's -2 ability will also give counters to all your creatures


  −2: Put a +1/+1 counter on each creature you control and a loyalty counter on each other planeswalker you control.


Abzan Ascendancy also gives +1/+1 counters to all creatures you have on the battlefield at that moment:


  When Abzan Ascendancy enters the battlefield, put a +1/+1 counter on each creature you control.


Archangel of Thune will work awesome with Abzan Battle Priest(the one you already have):


  Whenever you gain life, put a +1/+1 counter on each creature you control.


Borborygmos may also be used to get +1/+1 counters. And it's a big 6/7 Trample creature


  Whenever Borborygmos deals combat damage to a player, put a +1/+1 counter on each creature you control.


But Cathars' Crusade might actually be (one of the) best for your scenario. That card and Krenko, Mob Boss... I'm adding red to my Abzan deck!


  Whenever a creature enters the battlefield under your control, put a +1/+1 counter on each creature you control.


Celestial Ancient can also give +1/+1 counters to all your creatures:


  Whenever you cast an enchantment spell, put a +1/+1 counter on each creature you control.


Decree of Savagery for four +1/+1 counters


  Put four +1/+1 counters on each creature you control.


Mikaeus, the Lunarch a Legendary Human Cleric that will only cost you 2 mana to get a +1/+1 counter on all your creatures(Mikaeus might die in process)


  {T}, Remove a +1/+1 counter from Mikaeus: Put a +1/+1 counter on each other creature you control.


Phalanx Leader's Heroic ability can also give all your creatures a +1/+1 counter


  Heroic — Whenever you cast a spell that targets Phalanx Leader, put a +1/+1 counter on each creature you control.


Tempt With Glory


  Tempting offer — Put a +1/+1 counter on each creature you control. Each opponent may put a +1/+1 counter on each creature he or she controls. For each opponent who does, put a +1/+1 counter on each creature you control.


Titania's Boon


  Put a +1/+1 counter on each creature you control.


As Mephy pointed in his comment, Gavony Township can also be used to give all your creatures a +1/+1 counter.


  {2}{G}{W}, {T}: Put a +1/+1 counter on each creature you control.