Reed switches can be found very inexpensively on ebay. You want a pair of normally open type often abbreviated to 'NO'.
For a relay, you require a bistable unit. These can be a little trickier to find, but are available. You should aim to get one with voltage coils similar to your system voltage.
If you want to switch very high currents you can have the relay contact(s) pick-up the gate of a P-channel Mosfet to switch the load, or perhaps switch the low side with an N-channel Mosfet (anyone tried that?). To switch the high side with an N-channel you would need a driver chip to raise the gate about 10 volts.