You can get it here {its free} but you need to login.
http://www.macromedia.com/software/flash/download/detection_kit/
http://www.macromedia.com/devnet/logged_in/fsharples_detectionkit.html
I have also found places that have the code so if you are looking to learn about it. Here are a few sites I stumbled across.
http://www.actionscript4designers.com/wmg1/flash_detection.html
http://www.echoecho.com/flashbrowserdetection.htm
http://www.moock.org/webdesign/flash/detection/moockfpi/