Find historical stock prices for each firm from Yahoo! Finance (finance.yahoo.com). Enter the stock symbol, click "Historical Prices" in the left column, and enter the proper date range to cover the last day of the month corresponding to the date of each financial statement. Use the closing stock prices (not the adjusted close). To calculate the firm's market capitalization at each date, multiply the number of shares outstanding by the firm's historic stock price. You can find the number of shares by using "Basic" under "Weighted average shares outstanding" at the bottom of the Income Statement.