NIST no longer sends the Time and Frequency Bulletin by mail. However, the bulletin is produced monthly as a PDF document that can be downloaded here. The bulletins are available from January 1997 to the present but we only display back to 2013 here. If you need an older bulletin, please email kelsey.rodriguez [at] nist.gov (Kelsey Rodriguez).
The files are viewable using the Adobe Acrobat Reader.