Bison is available for most of the UNIX-like platforms and as the bison package from the Cygwin setup.

If GNU Bison isn't already installed or available as a package for your platform, you can get it at: http://www.gnu.org/software/bison/bison.html.

After correct installation, typing at the bash command line prompt:

$ bison --version

should result in something like:

bison (GNU Bison) 2.3
Written by Robert Corbett and Richard Stallman.

Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
      

However, the version string may vary.

A native Win32 yacc/bison package can be obtained from http://gnuwin32.sourceforge.net/. The installation should be straightforward.