The Federal Bureau of Prisons actually has an "inmate locator" where you can search inmate incarceration/release dates since 1982. You could print the record from this page and it might suffice for whatever substantiation issue you are facing. If you are trying to obtain records for litigation purposes, there are evidentiary issues with trying to admit printouts from a webpage. However, you could avoid these by asking the IRS attorney to stipulate to the document or to the dates of your incarceration. http://www.bop.gov/iloc2/LocateInmate.jsp
This response is for general informational purposes only. It does not create or imply an attorney-client relationship between the reader or any other person and Lapekas Law, P.A.. This response may not be relied upon to avoid civil penalties that may be imposed by the IRS regarding any matter discussed herein.