Disregard their comments regarding taper.

Shim stock can literally be *anything* that'll fill the space, be rigid, and maintain it's section.

In industry, shim stock is typically sheet in rolls or strips, often made of brass, stainless, and hard steel.
Your beverage can idea is a common source for shim material.
In the past we'd use the thin aluminum foil component of the Nestle Crunch chocolate bar wrapper. It was consistently .0002" thick. That's a fifth of a thousandth of an inch, useful in many situations.

An inch is 25.4mm
The difference between 25.4 and 26.0 is 0.6mm
0.6mm is 0.0236 inch
You need half that thickness for your shim stock.
I've used 0.010 with great success, that choice being driven by convenience. 0.010" (or "ten thou") shim stock is commonly found on the workbench or in the toolbox.

If you have a micrometer or a precision caliper, you will likely be able to find something around the house that'll do.
To be honest, the scope on one of my daily use ranch rifles features 3M Post-it Note paper as shim stock in one of the rings. It's working great - and I likely have whatever thickness of 'proper' shim stock in the shop -- furthermore my home shop is a machine shop which I could use to permanent rectify the situation, but the paper shim is doing very nicely.