Well Diva successfully ‘roped’ some yarn by interlacing it into a thicker cord, you can do that with string (cotton, NOT plastic).
Ironically actual bondage rope is usually a bit too thick to make a wearable crotch rope. You can do a simple single strand option, but if you want something fancy like the one on here (search crotch rope) you’ll need something with a smaller diameter.
Paracord is one of the simplest and cheapest things you can get, extra long bootlaces are great (CLEAN ONES), or have a look in a DIY store and see what you can find. In the UK we have stuff called sash cord that works well if it’s not too stiff (put it in the wash to soften and loosen any rope).
For the masochistic among you (I know who you are) try one of the natural ropes like hemp or sisal…and enjoy the suffering.
Remember you need way more than you think. The crotch rope I demonstrate on here requires twice your height!
And remember hygiene folks, chuck it in the washing machine regularly.