Hate to ask again-"official" source of the avg. currency conversion rate?

I hope this hasn't been done to death, but all of the posts I've found by searching seem to quote specific rates for specific years, but do not mention the source of those rates.

Is there a single "official" source of the average currency exchange rate over the year for people filling out the US tax forms?  Do you have to document your source and/or the rate you used?

I know of a handful of sites where you can get this kind of info, but in my experience they vary slightly depending on what they are trying to sell you. 

Oops--answered my own question.  But if it's not clear to anyone else, the Embassy page says to use:

http://london.usembassy.org.uk/irs/irsxchng.htm

The Embassy page in the link also specifically calls those rates "unofficial."

In my opinion this is a little pedantic.  The US and UK have not had "official" exchange rates since Sterling became a floating exchange rate in 1971 (until then it was fixed at $2.40).  So in practical terms the exchange rate is constantly floating; but the IRS generally and IRS auditors I have met are all content with any reasonable exchange rate.

The IRS accepts the Fed Reserve Bank exchange rates.  When these are not appropriate, they accept the rates found at reliable sources such as www.oanda.com.  The embassy site quotes the FRB rate.

FRB is at http://www.federalreserve.gov/RELEASES/g5a/.  I always use FRB for average exchange rates.  For spot rates and currancies not followed by the FRB, I use www.oanda.com