sorry this is such a basic question, but I can't seem to figure it out. Where can I buy Single Pull Double Throw (SPDT right?) relays?

 

have you tried:
www.digikey.com
www.newark.com
www.mouser.com

to name a few?

 

you might wanna check radioshack, any electronics store near you, auto supply store, etc...

also, you're looking for a single pole double throw. i'm sure if you said it fast enough, your way, they'd know what you meant. also, make sure it will handle the load you plan to put on it. a regular automotive relay should suffice. they handle up to 30 amps.