I have searched everywhere I cannot find a reference,
On the Bosch rear wheel sensor, there is a rubber stopper. Lost this stopper. Presently have a piece of tape covering the small hole.
Have not been able to find this part. Which seems odd, because I cannot be the only one.
Any assistance would be appreciated.

Possibly called a grommet, but wouldn't be surprised if you needed to purchase the entire sensor to get one. You might try to fashion one from a chunk of rubber and secure with a dab of silicone.

I have not seen that piece as a stand alone item unfortunately but I would reach out to Bosch or to your local Bosch dealer and see if they can help out:
https://www.bosch-ebike.com/en/service/contact

Sent a message to Bosch. My experience has been, sometimes they reply, sometimes they don't.

They have always replied to the shop and in the case they are late or miss it as they probably get a good volume of emails I just send a follow up and they respond.

Yup, just as you figured. Bosch does not supply a 50 cent stopper, you have to replace the whole sensor. A dime sized piece of gorilla tape will do just fine, until the sensor really needs to be replaced

I will take a look and see if I have anything in my stash of old parts. Usually they ask for warranty stuff back but on occasion they don't.

Thanks. If it bothers me too much, I'll just stop by he LBS where I bought the bike and see if they have any old sensors around. I've put up with it until now, I don't see any hurry to change it.