Here's one Ebay reference but the vendor seems to be temporarily out of stock:

https://www.ebay.com/itm/153595700092

For a temporary engine run, you can turn the rocker shafts upside down and pivot the rockers on the unworn top side. It's not a good fix, but it will allow you to make a test run. The rocker arm bores are probably egg-shaped as well but that's not going to keep the engine from running.

You could also try starting a thread in the "parts wanted" forum. Chances are that a stovebolter might have a trashed engine with a good rocker assembly on it. One was given away free just a few days ago.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!