The latest release (v2.4.1, released March 28, 2016) of simx is available here. Please note that this version must be configured with a --prefix command (configure --prefix=`pwd` will do fine). It also uses a different simx.par format, so you should remove any simx.par files in your pfiles directories before using it.
The final release of version 1 of simx (v1.3.6, released April 9, 2013) of simx is always available here. This version, however, is now deprecated and will no longer be updated.
If you need an earlier version for some reason, please contact the author, Randall Smith, at email@example.com.