The rates usually shown on sites like
http://www.xe.com and
http://finance.yahoo.com/m3?u are the interbank rates. Usually the only way to get an exchange rate (as an individual) that close to the interbank rates is to exchange dollar amounts of one million or more.
You're usually quoted lower rates by banks and such for lower amounts because that's how they make their money. The amounts quoted are usually (not always) no more than 2-4% less than the interbank rate.