eBay is where I have found them for as low as $20-35 each (though pay an extra $20 for each for shipping), but still works out to be a good deal..

I do have an extra Magnetek ballast, I need to verify the part number (which I have not done yet but will see if I blogged it or will get it this evening) if you are interested. Or I can see if the outlet I found my water proof Advance ballasts from still has any for you if you want to stay with Advance specificially. If so I'll drop you a PM with contact info.