I need to switch a submersible 120V fountain pump with a thermostat for a water-cooling application. My thermostat has a relay output, that I can hook up with 5V or 12V DC, 500mA or so. What's the best way to switch 120V? Another relay would be fine but I don't know where to find a 120V one. I have 12V 30A automotive relays; would that work with 120VAC ok?
Did you try searching at Digikey? I found models like this http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=PB323-ND
The "best" way to switch 120VAC depends on the current. Do you have any idea how much current your fountain pump draws? You could go with a "power" relay (as CrossRoads suggested) or for lower currents an optically isolated triac.
A 12VDC automotive relay is unlikely to be able to withstand 120V and may be destroyed.
-- The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O's fused and protected
One of these? http://www.mpja.com/prodinfo.asp?number=18703+RL