Warlocks do not get special class specific flying mounts. Instead, you can buy a faction specific flying mount from a vendor which should be located somewhere near the trainer who taught you flight.
For instance, in Stormwind, on the platform where the flight trainer is located, the mount vendor is the third NPC on your left (just past the flight master). He should offer griffons in a variety of colors.
For the sake of completeness: Here is a list of all Flying Mounts, click on any one to see where it is obtained. Most of those are Epic Flying mounts that will require you train 300 Flying at a cost of 5000 gold. For 225 (Expert) Riding, This is the entire list. Note that the Broom is a limited duration holiday item, the X-51 Nether Rocket is from the trading card game, and the remainder are faction specific.
Listed here, you'll find the 5 vendors that will sell the three mounts you can use right now. They're located in Stormwind, Dalaran, Honor Hold, Valiance Keep, and Wildhammer Stronghold in Shadowmoon Valley.