It is not often when one of our big investment databases such Mergent, Valueline, General Business File ASAP, and Business and Company Resource Center misses a subject area where a freebie comes in to play being historic daily stock price closes and stock splits. When someone sells a stock for tax purposes, and I make this disclaimer that I am not a tax advisor by any stretch and this should not be considered tax advice at all, a patron may seek the maket close price and if any splits occured (usually when 1 share becomes 2 to lower the price of the stock for buyers) to compute taxes. Keep in mind that this can get tricky really fast.
Long story short, finding the closing market price of a stock on a certain date can be tricky especially if it is from ~1999 or earlier as newspapers can be a good source or at the Main library Capital Changes Reports (thank you Amanda H for the other alternative). To find stock splits, reported market price close numbers the best site seems to be Yahoo Finance. Also the chart seen in the graphic is currently in beta version and is not the default.