The GPO publishes and distributes most official government publications. You can contact the GPO by calling the GPO Bookstore in Washington, D.C. (202-512-1800 or toll free at 866-512-1800).
To get GPO Publications: The GPO Web posts an online U.S. Government Bookstore. You should be able to order all of these through the Web site or by calling the GPO Bookstore in D.C. If speed is of the essence, the GPO warehouse is located in Laurel, MD, and you can hire a document retrieval service to make pick-ups.
In addition, the GPO gives its publications free to depository libraries that agree to make their collections open to the public. For materials published since 1994, search for the publication in the Catalog of U.S. Government Publications. When you find it, scroll to the bottom, click on the "Locate Libraries" link, and you can get contact information for the depository libraries that have what you're looking for.
If you need a pre-1994 publication, you can find local depository libraries using the Locate a Federal Depository Library database, then call or search the library's online catalog to see if they have what you need. Alternatively, you can search nationally using OCLC's WorldCat or by searching the online catalogs of other likely libraries.
Researching GPO Materials: The GPO's Monthly Catalog of United States Government Publications (MoCat) lists all government publications published by the GPO since about the end of the 19th Century. Though the issues come out monthly, the indexes are annual.
For materials published since 1994, you can search the online version of the Monthly Catalog, called the Catalog of U.S. Government Publications. Not only does this let you search all available years at once, the "Locate Libraries" links on the hit lists tells you which depository libraries hold each of the referenced publications.