As Mike9241 said, you should contact your broker to see if they have at least an estimate of your cost basis. If they cannot provide even an estimation, you will need to do a little research.
If the stocks you sold had reinvested dividends accumulate over the years it will behoove you to find as many of the quarterly and/or yearly statements you have received and can retrieve since you first owned it. Reinvested dividends are income in the year they were used to purchase additional shares of stock and add to your basis.
Also, if you know the approximate dates you purchased the stock that was sold, a website such as bigcharts.com can assist by giving you the price of the stock for the approximate date you acquired it. Google may also help with the stock price if, again, you know the approximate date you purchased it.
@k8t22