Are you buying a new or you buying a used? Cuz if you're spending the money to buy something new HPS is really not the way to go. There very expensive to run compared to other artificial light sources.l am leaning towards a 'Sun System Magnetic Trifeca' kit, it comes with a 1000w switchable ballast & a 'bat wing' reflector and a HPS bulb. I can then use the MH bulbs l got as needed during the future cycles. It's prices are all over the place, but l found a cheaper source in a B.C. gardening site that'll ship to me and address any issues l may have. The ventilation & filter will have to wait for a little while yet, eh?
So there's ones just like this on eBay with a lightbulb that'll screw right into your HPS reflector that you have right now for about a $135.00 bucks ballast socket adapter and a light bulb
Here I bought this one 2 months ago. It's on eBay right now.
It works ...Great