If your heli uses up to 8 cells and uses only analog servos, then you can use the BEC which is included on many ESCs such as the Schulze Future 12.46k and the Castle Creations Phoenix 35.
Some controllers can handle up to 10 cells with four servos, but 10 cells is very marginal for most linear BECs, and the BEC will run very hot. I recommend using an external switching BEC when running battery packs over 10 volts (more than 8 cells NiCD/NIMH, or more than 2s LiPo).
To use an external BEC, you will need either a free servo/battery position on your receiver or a Y-lead for the BEC's output power plug.
Currently there are several popular BECs. The first is the Kool Flight Systems Ultimate BEC, also called the UBEC. This is a large 20 gram BEC which can deliver 3 amps continuous. There are two models for 5 volt use, one for up to 29 cells and other for up to 36 cells.
The second is the Firmtronics SBEC. This is a very small and light (8 grams w/o switch) BEC which can deliver up to 2.5 amps continuous and can work with up to 40 cells.
There are now two more choices, which are the Medusa Research "Potencia" 2 amp/6-25 cell and 3.5 amp/10-33 cell BECs (12 grams). I have not seen any reports from people using these on helicopters, however.
Dimension Engineering manufactures two BECs which are usable for helicopters: the ParkBEC and the DE-SW050. The ParkBEC is rated for 1.25 amps and 33 volts of input and includes a built-in Y-lead. The DE-SW050 is rated for 1 amp and 30 volts of input and requires some extra soldering to use. Both the ParkBECs and the DE-SW050s can be wired in parallel to increase the current capacity of the units.
If you are using any digital servos, then you will need to check the ESC's onboard BEC amperage rating. Most ESC BECs are rated for only 1 or 1.5 amps which is insufficient to run a digital servo + 3 analog servos. One digital servo with three regular servos can easily draw well over one amp, so if your ESC's built-in BEC is rated for only one amp, you will definitely need to use an external BEC.
If you use a one amp BEC with digital servos, it will probably overheat during flight and shut down. This will cause you to lose control of the helicopter and it will crash.
To use the ParkBEC with an ESC with a built-in BEC, the throttle wire should be plugged into the ParkBEC header and not the receiver. This will automatically disable the ESC's built-in BEC.
To use the ParkBEC with an ESC without a built-in BEC, then both the ParkBEC and ESC must be plugged into the receiver (using a Y-lead if necessary). If the ESC is plugged into the ParkBEC, then the ESC will not receiver 5 volt power and will not initialize.
To use an (non-ParkBEC) external BEC with an ESC which already has a BEC, then you will need to disable the built-in BEC of the ESC. To do this, check if the ESC has one or two plugs which plug into the receiver. If the ESC has two plugs (like the Schulze Future 12.46e), then one of the plugs will have three wires and the other will have only two wires. The plug with only two wires is the BEC plug. If you do not connect this plug to the receiver, then the ESC's onboard BEC will be disabled. If the ESC has only one plug (like the Castle Creations Phoenix series) then the plug will have three wires and the middle wire should be red or orange.
Either pull this red wire out of the connector and tape it with electrical tape to prevent it from touching other wires or use a servo extension with the red wire pulled out and taped. This will disable the onboard BEC of this type of ESC.