You could possibly have a rubber stamp made up to make your own, or you might contact a local printer to see if they can do anything.
Years ago I
had a source for my cotton lined envelopes that used to offer printing on them for an added fee, but they are no longer in business.
I also made up an Excel template to print out my own envelopes via attaching the blank envelopes to a stiff paper backing and then feeding them through my laser printer. When my laser died I replaced it with an inkjet which couldn't
handle the thickness of the mounted envelopes.