I laser cut one out of a remnant delrin rod (purchased off rBay).
Nowadays I might try a 3D printed one by designing an SVG in InkScape, then importing that into TinkerCAD and extruding it before adding it to a background plate. Print it at high infill (80-90%) for heft. I made a clay stamp for my sister this way and it worked great.
Don’t forget to mirror your design: it will stamp “backwards” so text, etc. needs to be reversed.