Help with finding 385C letter of my business being a S-Corp

Hello-

I just started managing a small business in TN. For over 20 years the owners have been filing their taxes as a S-Corp. This past year the IRS now say the business was never accepted as a S-Corp. The owners accountant for most of those years has died a couple of years ago and the new accountant is asking for a 385C letter or any document(s) stating the business is a S-Corp.

Who would I contact to find any documents or info on this subject?

Thank you,

Robbie