Here's a 12 volt one that you could run down to Radio Shack and pick up for under $5.00. It would probably work with 6 volt since it doesn't take much juice to activate the electromagnet for a relay. The secondary (switched) side of the relay doesn't care about voltage since it's just a switch. If it doesn't work, just take it back.
http://www.radioshack.com/product/in...ductId=3020762