This is really simple! To get Mistletoe, just do the /kiss emote to a Winter Reveler. You won't always receive Mistletoe and can only do this once per hour, so try to save all the ones you receive. Here's the list of the "Brothers" you need to use it on:

Alliance

All the Alliance brothers are Human males. Let's start in Stormwind and work our way out to Nijel's Point, Desolace.

1. Brother Kristoff in Stormwind: Brother Kristoff stands in Cathedral Square. He's next to the statue in front of the Cathedral steps.
2. Brother Benjamin in Stormwind: Brother Benjamin patrols the right side of the Cathedral. He wears a white hood and robe.
3. Brother Crowley in Stormwind: Brother Crowley is located inside the catacombs. To access them, there is a corridor on the right side of the Cathedral just as you enter the main room. Follow it down and you will find him!
4. Brother Cassius in Stormwind: Brother Cassius is inside the room on the right side of the Cathedral. Also may be worth noting that he's a Reagent vendor.
5. Brother Joshua in Stormwind: Brother Joshua is the Priest at the Altar in the Cathedral.
6. Brother Wilhelm in Goldshire: He's the Paladin trainer at the back of the Blacksmith. Coordinates are 40,65.
7. Brother Nimetz in Stranglethorn Vale: Brother Nimetz is located at the Rebel Camp in Stranglethorn Vale, not Booty Bay. His coordinates are 37,3.
8. Brother Karman in Theramore: Brother Karman is located in Theramore, Dustwallow Marsh. He's the Paladin trainer on the right side of the Keep. Coordinates are 67,47.
9. Brother Anton in Nijel's Point: Brother Anton is located in Nijel's Point, Desolace. He's inside the Inn at the top of the hill. The coordinates are 66,7.

Horde

Let's start in Undercity and work our way out to Northrend. Note: You will need to be at least level 77 and have Cold Weather flying in order to reach the last Brother in Icecrown!

1. Brother Malach in the Undercity: Brother Malach is an Undead male that stands near the fighters in the War Quarter. Coordinates are 51,21.
2. Durkot Wolfbrother in Warsong Hold: Durkot Wolfbrother is an Orc male and is located inside Warsong Hold. Borean Tundra. He stands near the wolves at the stables. His coordinates are 40,54.
3. Brother Keltan in Icecrown: Brother Keltan is a Blood Elf male on the floating airship in Icecrown. He patrols around the boat so be patient while waiting for him.