As suggested a Castle BEC or any inexpensive UBEC ( make sure it says "U" or SWITCHING ) will do fine.

Analog BEC's tend to max out at 11.1v input voltage and even then they can only drive 2-3 servos.

This is because the BEC must shunt away the excesss current as heat... the more current, the more heat produced...

UBECs are SWITCHING MODE devices, that only grab bits of current by switching the voltage on and off very quickly, to power a reserve ( capacitor ) that produces the current to drive the electronics.

UBEC's typically can tolerate higher voltages and more current draw from your electronics.

You can find UBEC's from most online Hobby Sellers... Tower, Horizon, BPHobbies in the U.S.