If you are preparing a tax return for a deceased person, you may in fact have to go back to the account creation value for the cost basis of the shares, unless you have any better information. Where the dividends reinvested? If so, that would be added to the cost basis of the shares. You are limited to what you can actually establish as the cost basis. If you know when the shares were purchased, you might be able to establish basis from historical prices of those shares.
See this TurboTax Help Article.
If it's you that inherited the shares and then sold them, your cost basis is the fair market value of the shares as of the date of death.
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