AFAIK the lowest-visual-impact ladder line can not be bought already made but you have to make it yourself.
Use, say, 14 gauge bare solid wire (preferably hard-drawn) and put 4" polycarbonate spacers every foot or so with tie wires. www.cebik.com
has some details in the "Home brew parallel transmission lines" section.
I used 1/8" thick polycarbonate sheet and cut a bunch of 4" x 3/8" strips. Then I notched the strips at each end to fit 14 gauge bare solid wire (my table saw's blade happened to be just the right width) and drilled holes near each notch to put tie wires. I soldered my tie wires to the 14 gauge copper, but in retrospect just wrapping them would have been good enough.
Making it is a bit of a chore, I did 100 feet in my backyard taking up most of a Saturday morning, but it's beautiful after it's up in the sky, and yes it is a lot less ugly than brown window line.