Assuming that you are talking about either or both of the red and yellow parts (not the orange arrow one), it looks like a standard QR lever assembly.
One option is to slip the black collar off the bike and take it to a local bike shop. They should have some on hand for a dry fit test and/or measurement.
The inner threaded “screw” portion is almost always steel unless noted otherwise. The one you broke is like steel as well as the outer nut is a steel insert with plastic over-molded. Some of the older style QR’s have steel lever two (red section) so that could be worthwhile.
And as a guess, if this broke from heavy tightening force to prevent the seatpost from slipping down (seems a common issue with these bikes), I’d recommend using carbon fiber paste to improve grip without requiring large clamp forces.