Unfortunately financial institutions won't always have the acquisition info about securities you sell. If you know what you paid for the shares that were sold, you should just enter that amount*. If you don't know what you paid, but you know that they were purchased and when they were purchased, you can generally look up historic stock values and use that to determine what you paid. If you don't know any of that, then you'll generally just have to use any info you can to estimate when they would have been purchased, and conservatively estimate what you paid using the lowest market value in that time range.
*If the stock is "covered" stock, then you should report a cost basis of $0, then indicate you have "additional info" and use the "corrected cost basis" field to enter what you've determined as the cost basis. This is unlikely because typically institutions will know the cost basis for "covered" assets, and if not they will report $0. Also, if shares have split, or there have been mergers with other companies, it's possible you may need to adjust the original purchase price to account for these activities. Most historical price sites will show you a "split-adjusted" price on historical dates that account for share splits, so those are easy to account for. Other activities like mergers can be more difficult to account for, so hopefully that doesn't apply in this case.
