Well, given the necessary expertise, tools and material, you can make the rope you want! And that is, besides it being cheap, actually the best reason why you would want to make rope!
Traditionally Shibari practitioners prefer three strand twisted rope made from jute or hemp, that is 6 mm thick and 7 - 8 meters long. For special purposes, like tying toes, there is need for thinner (4mm) and shorter (2m) rope, and you might find other natural fibers interesting, like silk, linen and bamboo.
making a sample ropeEdit
If you aren't sure about the properties of the rope you are about to produce, it is a very good idea to make a sample, like half a meter. You can look at it, measure the diameter, do some stress test with it, and also test your future conditioning methods. Making a sample is recommended for all kind of doubts, like how many yarns to take, which way around, and how much to twist.