If you've lost your 501c letter and are trying to e-file, the quickest way to obtain this information is by making an appointment with your local IRS office. Due to covid, you do need to make an appointment ahead of time.
IRS - How To Make An Appointment At Your Local IRS Office
Please remember, this is truly for your safety as if you did receive such a letter to your home, it means that your address was used fraudulently.
