Do you suspect there is live conductors in the wall? Use this detector to make sure. As for the power source, it runs on batteries.
From ManoMano
AC-tiveFinder Black,Orange,White voltage tester screwdriver - Laserliner
Conrad Electronic
Laserliner AC-tiveFinder Non-contact voltage tester CAT III 1000 V LED
ITS
Laserliner 083010A Contactless Voltage Tester