I finally found a few boxes of .30 cal luger for sale on-line.
It's a
Maryland Munitions.
I talked to them on the phone to inquire and they only have a few boxes available.
It's expensive $75/box ($1.50/round), but at least they have it in stock. I haven't seen any for sale since 2020.
I had trouble ordering on-line, so I had to call them: