The home depot here carries 400 watt HPS bulbs. They just don't have the ballast's. Call them up and ask them and if they don't I bet a HD somewhere near you does. I just went to HD's website and searched for 400 watt sodium and the bulb came up. $45.96 a piece.

My rule is to match the bulb with the ballast. No exceptions. (unless it's a conversion bulb)

Laterz