Any regular hex nut works just fine as a jam nut. Basically, a jam nut is when you jam two nuts together. (It is gay, because the nuts do touch.)

And note that those nylon inserts kinda only work once. The bolt carves a thread into the insert when you insert it, so it will be weaker the second time you insert it.

Honorable mention: cage nuts. A square nut, permanently attached to a fastener that can snap into a special square hole in a 19 inch server rack. When you tighten the bolt against the nut, it tightens against the fastener, so that the nut, bolt, and fastener are secure against the square hole.