You find random pizza slices at the end of missions. As long as you have empty pizza boxes, the pizza will go in to a pizza box instead of being burned immediately.

Since Illegal Busking has the best XP to energy ratio of all the first level missions, I just kept doing that mission over and over again. With that many 5 minute missions, you run in to a piece of pizza pretty regularly I've found. But since it is all random, I could hit a dry spell where I run out of pizza and have to wait for my energy to build back up over time.