If you can't find a copy of your trade confirmation in your files, try contacting your broker to see oif he can send you the informatoin. If that doesn't work, you can estimate the purchase price per share using the average of the high and low price on the date you acquired the stock. Below is a link from Yahoo! to a listing of stock prices for BUD from 1996.
BUD: Historical Prices for ANHEUSER BUSCH COS INC - Yahoo! Finance (http://finance.yahoo.com/q/hp?s=BUD&a=00&b=1&c=1996&d=11&e=31&f=1996&g=d)
Remember that the stock split 2:1 on Sep 13, 1996 and Sep 19, 2000. So if you bought before Sep 13 1996 you divide your original purchase price per share by 4. If you bought between Sep 13 and Dec 13 1996, you divide the purchase price per shar by 2.