It is called a BEC and also sold as an UBEC and an SBEC. You can find them in most hobbyshops because they are well used on planes and boats.

Another more simple solution is use 2 diodes in the powerline. A normal diode takes 0.6~0.7v so with 2 you will lower the voltage with 1.2~1.4v

You can also use a LiFe receiver battery. then there is no need for lowering the voltage.