There are a number of online sources for the historic prices of stocks that are traded on the market. A search for "historical stock prices" will bring up options. For example, view the historical section at Marketwatch or Nasdaq, or Yahoo Finance.
If the stock is not sold on the market, but the company is still in operation, contact them directly for past price information.
You can also check old brokerage accounts or contact your broker if applicable.
See this TurboTax help article for more information.