You could try ebay. I know it may not be the best for name-brand mods, but for a box mod that somebody is probably building in their garage or basement, what's the difference? I just did a quick search and there are tons available for well under 100 bucks, many under $50. I kind of like the acrylic boxes.
Anyway, I recently got a gently used Hexohm from a member here and am glad I did, as I like the ability to adjust the voltage at the 510. I can't imagine having parallel mosfet box running full blast all the time. I know that can be managed by varying the resistance of the build - but I still like to have the option of changing the wattage on the fly using the potentiometer.