I'm not 100% sure, but I believe that is company Patrick was referring to (without calling out names) on the previous page.
Castle products are priced lower that most for the performance they offer in return. I think that if N50EH magnets were available, more companies would offer them or at least provide boastful specification to make up for the higher costs.
As well, you'd be able to buy them as raw material from places like this:
http://www.kjmagnetics.com/specs.asp
The best you can do for our intent and purposes (from this chart, at least) would be N38UH.