Any particular requirements or will any taxi do? There are a lot of them driving around town. Flag one down, get their card, set up a time to pick you up.
If you happen to be near the metro bus station when a bus comes in, taxis are usually swarming like mosquitoes, talk to one, get a card make a date.
If you were going to Haiti I can see a need to be more selective. Just to the airport, anyone should do and as I said, you can't walk very far without seeing one.