Radiator plugs for leak testing?
 

Are there rubber plugs to cover the inlet/outlet to test for core/tank leaks?

Try local commercial ac electrical ac plumbing supplier.

Are there any radiator repair shops near you?
They'll should have them.
Only one or two shops left here long lake Michigan from I'll to Michigan city steel mills.
Used be 5-6 in both countries.

I've used 2 radiator hoses with a coupler connecting them in the past.

Probably the easiest to chase up!
Did find these
https://www.pegasusautoracing.com/pr...RoC7d4QAvD_BwE

Skip, formerly I've used a piece of old inner tube with a hose clamp over the neck, when you're poor, you find stuff lying around the garage to use.................;)

I still have inner tube and hose clamps lying around my garage, probably still plug it that way today.............LOL

:2cents:

That would be an easy way!