To find food trucks/carts in a new city, how about using an iPhone or Android app? I've got two loaded up on my Android phone: Food Truck Follower and Eat St (Food Network Canada). The Eat street app maps vendors on Google maps which is good for carts and trucks with regular locations but obviously not so great for roaming trucks.

The Portland foodcart website listed previously is really good. We've also used Chowhound to help identify pods and the better carts.