Ron,
Normally 6GA would be minimum for 220V/50A and depending on the length of run 4GA is probably better (anything over 25 ft I'd go 4 GA). You'll likely struggle to find a ready made extension cord, best bet maybe to go to a home depot/electrical wholesaler and purchase wire off the roll and the correct plugs. Make sure you get a multi strand copper wire (not aluminum), trouble is all this may ending costing nearly as much as the proper welding cable (which is usually very fine strands of copper for the high current demand application)
Good luck,
Julian
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