Should you find yourself stationed in an urban environment, you probably lucked out in terms of acquiring food. People have food everywhere so, since I'm assuming the population has decreased significantly, you're going to have some options. However, depending on your disaster and specific location, certain options may or may not be more fruitful. For example, if people had time to prepare for the disaster, there will be both more people still around and less food in residencies. In that case, look for grocery stores to take food from. Remember, fresh produce, meat, and milk are the first foods to go bad, so make sure you either eat them first or find somewhere where you can store them for some length of time because the grocery store most likely will not have electric power. If there is not a noticeable number of people still in the area, I would recommend staying in the grocery store. The reason I don't recommend it when people are around is that they will probably converge there as well, and if you take residence there you may be perceived as a threat. But I'll leave social interaction for another post.
A key to remember, especially in an urban setting: know what is worth the effort! Don't risk the dangers of urban decay for minimal or dimly-perceived rewards. Only spend effort on something if you will be rewarded with more energy than you spent. As a corollary to that: don't unnecessarily risk your life! It's your biggest asset! If you're not sure if you should do something risky or move on, remember the rule of threes. An average human can survive for about three minutes without air, three hours without protection from the elements, three days without water, and three weeks without food. It varies slightly per person, but that is a good guide. Keep it in mind if you happen to need to decide between scaling the side of a building and going a few days without food. Your well-being will thank you.