D8R, battle switches can be found here.
http://www.dimensionengineering.com/battleswitch.htm
A lot of different hobbiest electronic suppliers have similar devices, that go by different names, but they are all basically a miniature relay, designed to be activated 'on' by a low current signal, and used to supply higher current draws to bigger loads.
As for back-up beepers or alarms, I don't know of anyone who offers a stand along back-up beeper, but many ESC, combination sound units have this feature built right in. Servonaut comes to mind.