Good question.
If you know the # of shares and when purchased, you can go back and find out the closing price that day and multiply for the basis. This may not get you an exact basis, but it'll be close.
The truth is that the IRS really doesn't or can't care what this figure is, only that the relevant equities were purchased far enough in the past to qualify for this treatment (as well as what its value was when it was donated, of course).
What's unfortunate is that some charities don't make it clear that there is a holding period... thus some folks donate appreciated stock that they haven't held long and lose the advantage of this provision of the tax law.
