Where can I find string that doesn't stretch for physics? And what is it called?

Haha. We call it an "ideal string" with ideal meaning that it has no mass and does not stretch. Of course, no such thing exists. In real life everything will stretch when put under tension. You can find things with larger Young's moduli but you can never find a material with an infinite Young's modulus.

Floss? twine? what is this for? or is this a riddle? I don't fully understand the question