Any AC-USB charger with a micro -USB connector that puts out 5 volts no more) and above 350mA will work. Higher mA ratiings will charge it a bit faster. You don’t have to worry about burning it out (unless you go over the 5 volts); the player has built-in circuitry to prevent that; it will only take what it can handle.
Many newer cell phone chargers will work. 
