I've got the same thing on my fresh water top off setup. Here is the solenoid that I bought.
http://cgi.ebay.ca/1-4-Electric-Sole...item45f78bcc35
To wire it I used a 12VDC adapter which used to be on our old internet router but you can probably find one for $5 at princess auto or something. There are two wires on the solenoid and two wires on the adapter. So just cut and connect them with two butt connectors. Pretty easy.
Here is a picture of where I have it. The garbage can has a float valve inside to keep it from overflowing and the solenoid is on a timer (ACIII) which turns it on for an hour each day.
