If you can charge at 240V instead of 120V, that cuts the current drawn from the outlet over the cord in half. So you can use a longer and/or lighter weight and cheaper extension cord. Just about any quality extension cord under 100 feet that sells in the US for about $10-15 can handle 7-8 amps. For my electric lawn mower, and leaf and snow blowers I use such 100 feet cord good for up to 15A and they only draw about 10A or so. No problems at all.
With my Vectrix, I can adjust the charge current easily. From 5A to 15A or so. I carry a light and relatively cheap extension cord with me under the seat. It has saved me several times where I can't park close enough to an outlet to charge without an extension cord and I'm running out of battery. I simply dial down the charge power to say 800 or 1000 Watts, so the current is something like 7-8A, and that light extension cord I carry with me works wonderfully on regular 120V in the US. It is outdoor rated orange type, just not as beefy, expensive, and heavy as a 20A one would be. It gets uncomfortably warm near the plug at full power though, even though it is rated at I think 15A - it just is not well made, so I never use it for more than 8A or so. It won't burn at 15A but it gets pretty warm near the plug (not elsewhere).