You can get chargers the following way:
You can look for the chargers in the buildings. Go to the building and tap the Level Info button. Level Info shows the information about the collection items and chargers you may find in this building.
Go to the collection and tap the charger you have not got enough. If you choose Find you will be prompted to the required spot in the city where you can find this charger.
You need chargers to complete collections.