Great question, and actually, it’s a bit of a misnomer. Crotch rope doesn’t often use rope at all! You certainly can’t do much with a thick rope, apart from run it through your legs once and possibly add a knot near your clit.
What you’ll see on my crotch rope instructions (holy shizzle sticks, that’s got nearly 2,500 notes…) is that some kind of cord is really what works best:
http://female-orgasm-denial.tumblr.com/post/121499270313/the-crotch-rope-step-by-step-this-is-my
So there’s lots of options for a soft,fibre cord. Long bootlaces can be a great option but paracord, available from most big stores, is a great solution. It’s a little on the thin side but shop around, or visit a DIY store and see what they have.
What you want to avoid is anything that doesn’t have any give in it, like that plasticky string, that’ll be awful.
Remember for a crotch rope as above you want TWICE YOUR HEIGHT (I know right…) in length, but you can do simpler ones with less.