A piece of pistol brass is certainly the quickest way to get started on this. Any piece of tubing of the right size will work though.
I like to make my headspace measurement bushings from cut-off barrel sections, and mark them for the case they are intended to measure. No particular reason for using barrel sections, except I already make a similar bullet ogive measuring tool using them.
If you're not familiar how to measure shoulder bump-
Zero caliper on a fired case (check with several):
Measure shoulder bump on sized cases: