The 2 oz bottles of Stan's tire sealant come with a "spout cap" that doesn't seem very air tight. It's intended for single use but I wanted to use 1 oz and save one for later.
A few people online say they've re-used those bottles by putting plastic wrap over the top before screwing the cap on. I don't really want to risk it leaking in my saddle bag tho, and I really don't want to find the bottle dried out when I need it.
The caps from 2 oz plastic vanilla extract bottles fit perfectly.
The cap from the 4 oz glass bottle on the far left in the first picture is larger, but the other three caps are interchangeable. There are surely other products sold in 2 oz bottles that would also have interchangeable caps.
Stan's probably buys their spout caps from someplace like The Cary Company (bottles too), and they sell normal caps for 15.1 cents. It's a great deal until you find out that shipping for one cap is $9.35.
These caps have a nominal diameter of 20mm and the threads are style 410 (I looked up how to measure plastic bottle caps). They're available from numerous online sources but it might take some searching to find free or at least reasonable shipping.