Question on mogul sockets

I am attempting to locate a S51 mogul socket for a 400 watt HPS system. In my searches I notice E39 sockets are 400 watt mogul sockets. Are these interchangable? The same thing?

Also, what is a good online source to purchase the S51 socket (or the E39 if they are the same)?

Doing further research and I guess i am begining to answer my own questions. S51 is the ballast (designates a 400 watt HPS ballast). So I guess the only question that now remains is the type of socket I need and a good place to purchase them.