Lowes. Home Depot. Mennard's and some smaller hardware stores have 12/3 extension cords with 15 amp plugs and outlets (why they don't do a 20 I don't know) Even found one at Ollie's (Great Stuff Cheap)
My favorite I got at either Sears or K-Mart (but they are getting hard to find) this one has a push button on the outlet end. it grabs hold of the dogbone's plug and holds it in tight so it does not accidentally come out.
They come in assorted lenghts.
BUT.. when I was having my engine replaced I used this cord (100') to power just my converter. NOTHING ELSE in fact the converter is a plug in unit.. draws about 10 amps at max draw.. the Mechanic reported the plug end of the cord was a tad warm. not hot. just noticeably warmer than the surround. And so it was.
and like I said that was 10 amp max. if I needed more I fired up Mr. Onan.