If you are going with 6S, the $17 Motrolfly U-BEC works great right off your main power battery. Deans are already attached, so it's plug and play... bottom of this page:
If you are going more than 6S, this Pro version work great and can pull right off your main battery packs too, or feed a separate battery to it.
Or if 6v is enough for your servos, then you can plug this, or any equivalent, right into your Rx:
No internal BEC is needed in these cases.