I may be wrong, but my memory is hazily telling me that "Super ends" are 3Rensho brand of socketed dropouts (hence they're found on 3Rensho and Makinos). But other NJS frames have similar ends so there probably are a bunch of manufactures.

I know you can get Kalavinka track ends shipped to the US with ease, but not sure if they have external socket ones. See http://www.kalavinka-bikes.com (though its not working for me right now).

Theres a really helpful Japanese bloke who posts in the Track and Singlespeed/Fixed Gear forums called Tomity. I'd speak to him as he probably knows exactly where to get this type of track end.

Tomity's website is here: http://homepage.mac.com/tomity

Or just post a request in the Single speed/fixed gear forum. Theres quite a few very knowledgeable folks on NJS lugs/frames there.