The Galbert Caliper is out of stock until next year.
The only other tool that I can find is the Robert Sorby Sizing Tool for Tenons and Spindles.

I could swear that I've seen another tool that seemed like a combination of the two. It looked like the Sorby sizing tool but it had a dial on it to read the current size while you were cutting.

Do you know of any alternatives to those two tools?