Technically, you could easily make your own. Since you are only interested in getting the viscosity right for YOUR situation, you don't need it to match what the rest of the world is using.

Get a small container that is shaped like a test tube (one of those metal cases that some cigars come in would work)

Cut it about 1" long and glue a stick to the outside for dipping, then drill about a 5/32" hole in the rounded bottom.

Dip it in the paint, pull it out and measure the seconds it takes to reach the bottom (Don't count residual drainage)

Let's say it takes 8 seconds

If the paint it too thick, thin it out a little, measure again - maybe it's 6 seconds this time.

If the paint works well, just use 6 seconds as a target.