Back to the seat post..
Shim stock in brass is made in a variety of thicknesses
measured in thousandths of an inch.
using the stuff is a machine normal practice,
I've seen it packaged in the Model Maker's hobby shops too..

Up your ability to measure things with a digital, or dial vernier caliper..

Harumph, next you're going to tell me to trade in my slide-rule for doing calculations.

I'll stop by the hardware store on the way home and see what they got.