A good spot for them is the Western Plaguelands. The area south of Andorhal is full of skeletons and zombies that are relatively easy to solo for all classes.
Whats better, they drop Major Healing potions. Handy if youre a melee character. Furthermore, you can start working on your Argent Dawn faction here and get yourself up to a point where they vendor at Chillwind Camp will sell you Enriched Manna Biscuits, these are one of the best sources of mana regeneration in the game. A nice addition to sell WOW Gold.
You can go from 50-53 here if youre in the mood, if youre not good at taking even level mobs fast then another good spot is Tanaris where there is still a ton of 46-49 mobs floating around the South West part of the zone. The Ogres in the camps there are probably best as they dont run at low heatlh and the respawns are a good speed.