You can download the upgrade file for 5.1.1b from http://www.cisco.com/cgi-bin/tablebuild.pl/callmgr-51
The files are:
1. ciscocm-5-1-1b-ReadMe.pdf
Readme for Cisco Unified Communications Manager 5.1(1b)
2. cisco-ipt-k9-patch5.1.1.3000-5.tar.gz.sgn
Cisco Unified Communications Manager 5.1(1b)
I'd recommend installing the latest DevPack as well. You can get it from the same link