Look into farm supplers for silo unloaders.
The ones I see have ruggedised 6/50 fittings.
We have one here salvaged from a silo unloader setup, all I did was take the plug from the unloader, and attach it to the other end of the 125 ft 6/3 SO cord (which was hard wired into the cotactor), and voila, welder extension.
(has also been used to power a seed cleaner setup too).
Our cord has the ones at:
http://www.khindustries.com/products/plugs-conns.html I suppose any good electrical supply house would be able to get them.
