It was originally using a bluewave 7 ballast which runs 2x250w. I bought this unit as a package deal specifically to get the ballast.
You can buy the exact same thing (will plug right into it) from Dave here:
http://www.canreef.com/vbulletin/showthread.php?t=80221
Or if you'd prefer to buy electronic ballasts you can find some used as well I'm sure. These will need to be spliced yourself unless the newer electronic sunlight supply ones have the same connectors.