How do I find a stock's cost basis if the stock was purchased as part of an Employee Stock Purchase Plan ESPP? What justification?

Hello, I have purchased a stock through an Employee Stock Purchase Program (ESPP) over many years. I ended up selling but the stock purchases were so old that my broker statement says Undetermined Term, B or E (basis not reported to the IRS). How do I find the cost basis? What justification, documentation does IRS require? Thank you!