The ballast for that light is inside the light housing.
That light is a peice of junk in all honesty,i understand being on a budget/tight on cash or simply not wanting to invest large amounts of cash into lighting but you can do much better than that peice of crap for under $50 & have your choice of High Pressure Sodium or Metal halide.
When your on ebay use the main search bar & enter "HIGH BAY LIGHTING" or "LOW BAY LIGHTING" as seperate search terms,what these searches will show is used & new warehouse lighting,high bay & low bay are allmost allways hps or mh with the vast majority being 400 watt models,what seperates high bay from low bay is the reflector & nothing more,both will work perfect for indoor grow op's.
Here you go with some links,keep in mind that most guys selling used high/low bay lights are very willing to cut deals for in person cash purchases if you dont go through ebay,email a seller in your area & pick up in person.
High bay.
http://shop.ebay.com/?_from=R40&_trksid=m38&_nkw=high+bay+lighting
Low bay.
http://shop.ebay.com/items/?_nkw=low+bay+lights&_sacat=0&_fromfsb=&_trksid=m270.l1311&_odkw=low+bay+lighting&_osacat=0
These are the lights im talking about,you can pick them up all day long for about $25 each & the ballast is much easier to turn into a remote ballast.