I suspect that the seller of the welding machine will have extensions. Failing that you could build it yourself. Wire insulation that I have seen is all good for 600V. 20 amps would call for #12 wire for reasonable distance (say 50 ft). 4 conductor cable gives you two hots, neutral and ground (the ground can be smaller if the cable comes that way)
Bill
I have the 175 and love it. If you're going to do thicker material, then go to the 210. If mobility is important and you're not going to be doing a lot of stuff over say 3/8" then the 175 would be great and you can still lift it by hand to move it if need be. I am very happy with mine.
I think Bill has the best idea on the extension, just go to the hardware store, get the wire & plugs and make one yourself. If you could actually find one already made it would be a fortune!