6 pack of stainless steel cable chain. Sturdy and strong with lobster claw clasp.

Size: about 17 3/4 inches long; links: 2x1.5x0.2mm

I usually add an extenter to the end. Those are available here:

https://www.supplyfordiy.com/shopnow/extender-for-necklace-making-with-tiny-heart