The Wyrmrest accord in Dragonblight - Reputation, sells a red dragon sells for not that much, few 100 gold, I forgot the price exactly.
The Dungeon "Culling of Stratholme" has a Bronze Dragon with 100% drop rate on a bonus boss (right before the last boss, Mal Ganis)
The Oracles - Reputation, Collect the Green proto drake (also one of my favorites) from the egg wich has a 7 days hatch time. I got this 6 times before all in a range from 2 weeks to 6 month's, all dependent on your luck.
The Dungeon - Utgarde Pinnacle, has a low chanse to drop the mount Blue Proto Drake from the boss wich rides the drake himself.
Time lost proto drake, is an open world spawn in Storm peaks, Wouldn't call this easy to get though since a lot of people try to farm it all the time, but its a very rare mount to have, with a few days respawn timer (or after a server crash?)