The kind of rope you use depends on applications. Hemp is good strong rope, but can be a bit itchy, cotton is smooth, and will not cause rope burns nearly as easy, but is more stretchy. Most tack shops (place where you buy stuff for horses and such) will have various variaties of rope at reasonable prices. They also have an excelent choice of crops and whips. I would recomend buying different kinds of rope, and seeing what one suits your puropse best. My guess is your going to have rope running between your legs and up your ass, so try the rope out in the store in the crook of your elbow, when you bend your arm. See how it feels.

I will be interested to see what you say worked best.