Advice on relay set-up
There are plenty of 5V relays available.  The trick is getting ones that switch a high enough current.

If you can't find one, then use a 12V one and power it with 12V.

Whether you're using a 5V or a 12V relay you'll require something between the Netduino and the relay to be able to provide enough current to switch the relay - be that a transistor, or a chip like the ULN2803.  Either of these can switch 12V as easily as 5V.

If you want to use the ULN2803 with the Arduino/Netduino I have designed a handy shield you can etch yourself with Toner Transfer.

The other option (which will be much quieter, albeit at more cost) would be to use a "Solid State Relay".  This is essentially an opto-coupled triac which does the same job as a relay but without any moving parts (except photons).  These, being opto-coupled, use about the same current to switch as an LED uses to light up, so can be linked direct to the Arduino/Netduino with nothing more than the normal current limiting resistor for an LED.



Added RM:

Majenko's 'shield' implements this circuit. Its a good useful universal driver circuit. Note the connection of pin 9 to V+. This connects the on chip catch diodes to supply to dissipate stored energy in the relay coils when they are turned off. If you don't make this connection you will end up with an exciting high voltage generator - possibly a short lived one.
 (Circuit from(here) which does not give any other detail.