You will have to contact the financial institution handling your stock account and see if they can help. The burden of proof is on you to prove the cost price should IRS need information.
If you know the year you purchased, you may want to determine the price based on some such information such as payment from bank account etc..
Let me know if you have any question. Bonus and Feedback will be highly appreciated!!!
Please note: This advice is provided with the understanding that all the relevant facts have been provided by you. Any change in facts might affect the advice given and hence may not be relied on in such cases.
Certified Public Accountant (CPA)
CPA, MBA, Over 10 yrs of experience in tax planning and business consulting..