IRS Publication 1542 used to talk about the rates, but per diem rates can be found at the U.S. General Services Administration (GSA) website. Per diem rates are used for travel within the continental United States.
When visiting the GSA website, taxpayers will find a series of search boxes that allow a search based on city, state, zip code and fiscal year. It is important to note that per diem rates prior to 2014 are not available for any city or state. There is also a downloadable PDF file available on the website for those who prefer searching at their leisure.