I worked for Conoco for many years. During this time I purchased Conoco stock as part of my company thrift plan. Conoco merged with Phillips to become ConocoPhillips (COP), and I then continued to receive COP stock, plus my Conoco stock was turned into COP stock. Over the years my stock was moved from broker to broker by Conoco and ConocoPhillips. I was eventually laid off by COP, and I then consolidated all my COP stock with Vanguard - some IRA, some non-IRA.Now I would like to sell all my COP stock, but I need to know the cost basis of the stock, at least on average. I have lost all the paperwork my company gave me when it bought stock, so I have to estimate something. I have been unable to find on the internet any price history for Conoco stock.My questions:1. Where can I find a listing of Conoco stock prices from 1975 until it merged with Phillips August 2002, ideally expressed as monthly or yearly averages?2. Are there any recommended strategies accepted by the IRS for estimating the cost basis of stocks when purchase records have been lost?