I'm into sailing more than B&D, but just a word of advice for those going to marine stores for rope...
There is such a wide selection of rope available, that you may be overwhelmed by all the pretty colors and styles. However, I believe it is important that you ONLY buy nylon rope. Nylon stretches a bit, whereas dacron and most of the newer synthetics (dyeema, etc) are specifically made to NOT stretch.
Nylon is usually sold as mooring and anchoring lines. You can still get a variety of colors, diameters, and styles (twisted, single-braid, double-braid...), especially in mooring lines. The bonus is that it's usually the 2nd-cheapest rope, after [slippery and mostly useless] polyethylene.
Interesting that you feel that way. We have plenty of nylon rope, but I really like the high tech rope used for sailing that doesn't stretch, in fact, if you pull the rope tight horizontally and let go of one end, it will not droop--it will stay up in space. I wish I remembered what material that was. EDIT: 10/1/18 -- I just found the spool and it says "Yale Cordage Racing Braid"
Regarding stretch, it is unlikely a human will get 1/4" or larger (we have up to 5/8" diameter rope) to stretch once they are restrained. I probably use 3/8" the most often. We have about 600 feet of rope in different lengths of braided and double braided rope. I don't like twisted rope because it isn't smooth.
A word about rope hygiene: since it frequently rubs against the vagina and anus, especially when formed into a harness, it should be washed in the washing machine if it becomes soiled with vaginal secretions, blood or feces.