You can find the cost basis for your shares with reinvested dividends in the following ways:
Check your brokerage statements or records from the company. These documents often track the cost basis of your reinvested dividends over time.
Contact your broker for assistance. They may have detailed records or tools to help you determine your cost basis.
Go online for historical stock prices For example, view the historical section at Marketwatch or Nasdaq. It's generally acceptable to take the lowest and highest price from a given day and average them to arrive at a cost. It's also acceptable to use the closing price on the day of purchase