| Talk to your Ameritrade Representative, and they should be able to help you track down the cost basis information. If that approach does not yield a result, then perhaps the next best thing is check your old statements and try and use the cost basis from the trade settlement of the stock trades.
If you cannot locate this in any of your prior statements then I suggest that you "use your best estimate", it must reflect a price that was an average price for the security in the year of the purchase. You should also keep a record of how you arrived at the cost basis of the underlying security that you purchased, that is keep a copy of the source document. | 
