I ran up and down the straight streets in NY and found 3 in about 40 minutes. The frustrating things was that it seemed each time one showed up he lead me into a zone where guards start chasing you too. Made it pretty tough catching him but I pulled it off. Those couriers are fast in this game.
Make sure you are unarmed and push B to tackle him, otherwise you'll have to do it all over again.
There's a small chance of one spawning whenever new citizens are spawned, so what you need to do is do something that causes lots of citizens to spawn, such as hanging around crossroads, throwing money on the ground or going to maximum notoriety.
Bear in mind that you have to tackle them with B, not assassinate them with X.
But you can use the rope darts to slow them down, or if you want to chance it you can use a poison dart, but if you do that you need to tackle them quickly before they die.