the new law will require brokage firm to provide your base in the future to both you and IRS. I think too many people under reporte their bases. However, for the old one, you may just have to check with brokage firm or based on best of your knowledge.
我也是用的DATEK. Datek was bought by Ameritrade in 2002. Ameritrade should have all of your Datek transaction. Log into your Ameritrade account. Click Tab "Account" ->"History & Statement". choose a date range such as 01/01/2000 - 12/31/2000. you can see transaction type "Sell" or "Buy" or all transaction type. you can see your records from 04/01/1999 to 02/12/2000, 01/01/2011 - 04/12/2011 as long as the range from start date to end date is less than or equal to 1 year.