You need to enter a reasonable and good-faith estimate of cost basis.  The IRS is receiving the 1099-B information from the broker, but is relying on you to report the correct category and/or cost basis. Otherwise the entire proceeds will be considered gain.

(It would be unreasonable for the IRS to claim you did not pay anything for the investment (Cost basis = "0", total proceeds taxable). It would be unreasonable for you to claim the total proceeds as the cost basis (no taxable income). 

 Historical prices can be found at: http://finance.yahoo.com

Also, consider the possible effect of re-invested dividends. These would have been taxable at the time you received and reinvested them, and would add to the cost basis.