I have an envirolite myself, only a 200 Watter but in a small area big enough for two plants it's doing the job perfectly.
It is as Toke said self ballasted meaning you buy the whole kit. The one that I bought contained a large 200W envirolite bulb and a reflector and ballast (bulb holder) with a plug attached.
I purchased a hanging kit too which was well worth the extra few quid to have the light suspended.
You need an E40 Edison screw fitting lamp holder, which 'most' reflectors have, but they don't use a separate ballast, they're self ballasting and igniting.