Well, the short and unhelpful answer is "you figure it out." If you at least have a known starting point all the information you need is available somewhere and there are firms that will figure out your basis for a fee. If you know at least the year of your purchase, haven't had additional purchases along the way other than the DRIP and haven't sold any of the stock until 2015, you can even work backwards to an acceptable estimate.
You certainly want to use the most accurate figure you can although as a practical matter you can also use $0 as the basis, too. Before you go to a lot of effort, and maybe cost, you might try entering $0 as the basis to see what the tax impact is. If you happen to be in the lower 2 tax brackets long term capital gains are taxed at 0% at the federal level so it's really not worth the effort to come up with a "better" number. Even if you're out of the bottom 2 brackets the next rate used is 15%.
Tom Young
