There hasn't been one available.
Most ball and cup adjustable pushrods are 5/16" ball and cup. You can get away with using one of those if you account for that difference in the measurement.
You can make one out of a tubular pushrod. You may or may not be able to do it with the tools you have at home. You need to cut one close to the top, thread the cut end of each section, install threaded rod in one end with loctite, no loctite on the other so the halves can be threaded together by hand.
You can measure the pushrod with standard tools that someone in the hobby has along with a 5/16" bearing. Set a combination square to about half the length of the pushrod, measure that with a caliper. Then hold the pushrod on the combination square, with the bearing in the cup and measure from the end of the rule of the combination square to the end of the pushrod. Add those together and subtract 5/16" and that is the length you need. Pushrods don't require as high a degree of accuracy as other parts of the engine.so this method, while a little clunky, works fine. A few thousandths of error won't matter here, like it would with pistons, for example.
If you want to make measuring a little easier, get a 12" caliper. You can find quality used ones pretty cheap. Vernier tend to be cheaper but you need your glasses, good light and a degree of skill to read them accurately. Dial and digital are much easier to read but tend to be more expensive. I prefer dial over digital because there is no battery to die.
I got a used Starrett 12" vernier caliper a couple years ago for under $100, for this purpose.
Lawrenceville, GA