SEarch on this site for MH balast.
I have a 71a3802 I think it is Advance brand, and I bought it at my local Graybar dealer for around $40. it powers a single MH 250 watt...XM or Iwasaki only (from what I understand.) I am told that this is a Mercury Vapor ballast, so it may not power a Ushio or other bulb.