Well, I found a source for the thick rope.
For anyone else who is interested , you can get 3/8" and 5/8" thick cotton rope from Empire Magic http://www.empiremagic.com/docs/rope20.html
They call it Deluxe Euro Rope.
"EuroRope is larger than most regular magician's rope which measures 1/4" inch in diameter. Deluxe EuroRope measures 3/8" inch. Super Deluxe EuroRope measures