The Accurate Shooter article is at https://bulletin.accurateshooter.com/2025/01/use-tall-target-test-to-verify-your-scopes-true-click-values/ It describes their use and provides a worksheet (link).

These folks sell one but its expensive – https://www.dusupply.com/extra-tall-target-mil-tall-target-test.html?srsltid=AfmBOoqnl7wC6PXEkpNWTdz53-DHqKewH5jS_DJnz8vPJPlCNib_bUW_

These folks offer a much smaller version at a much lower price (as of 05312026 they are “out of stock”, so availability needs to be confirmed) – https://cerusgear.com/collections/targets/products/scope-tracking-and-load-validation-target?variant=42055122518270

NOTE: Check the target material of the above compared to what is described in this article.