generally, any USB charger will work. The difference is in the output the charger can do. a normal USB 2.0 port will do 500mA, some cheap chargers will do 500 or 700mA, I've also seen a couple at 900. most phone chargers are 1.0A, though the stock N4 one is 1.2A. Most Tablet chargers are 2.0 or 2.1A.
Higher number = faster charging. Most devices are smart enough now to not charge so fast that it'll damage the battery. Generally if you want to charge at any speed you want to go at at least 1.0A. 500mA will work as long as the phone isn't seeing heavy use, but it'll be slow. in some cases, a phone or tablet can draw more power than a 500mA charger can provide, and the battery will drain even when plugged in.