I have considered buying an electric chainsaw, but I don’t know where to get an extension cord 500 yards long.
It's called a "battery".
If 500 yards is literally an issue for you, though, it's probably a bad idea. I own a modest handful of acres, and every tool from string trimmer to tractor burns gasoline. Batteries just can't keep up. Trying to do real farming with them is beyond insane.
Just carry a generator and a gas can with you!
Battery powered chainsaws are great for personal yard use.