Any USB charger should be safe to use, except for "fake" ones like this:
http://hackaday.com/2012/08/15/buyin...-saving-money/
EDIT: There was a warning posted by knc1 about a K3 firmware bug, where a charger that was "too powerful" could damage the K3 charger circuit because it did not properly limit the current. But 700mA should be safe (it is well below the 850mA specified for the K3).