Rates of Exchange - Annual Average Rates
IRS does not post an exchange rate schedule on its web site. Instead, you may use the US Treasury rate from Financial Management Service (FMS).
Other options: www.oanda.com provides exchange rates. People using that site should obtain the average exchange rate using the 'interbank rate' for the period required, for example, January 1, 2004 through December 31, 2004. You input that information and the site automatically gives you the average rate for that period for any currency in the world.
If you go to www.newyorkfed.org, you can download the daily rates into an Excel spreadsheet and have the spreadsheet compute the average rate.
Any of these rates could be used and accepted. If the IRS were to examine the return and ask how the exchange rate was computed, the individual would need to be able to show how the rate they used was computed. Since any of these are obtained from legitimate sites, they would be accepted.