It looks like they rolled their own. Based only on CSS class names and a
quick Google search, maybe they copied it from this blog:
It's pretty simple. It's CSS3 animation/transitions. It
won't work on older browsers, but most sites like this don't
care. Windows XP users will appreciate not having to waste CPU cycles
on slides.
Many are out there on live sites but I can't find any
source files to save my life.
You didn't look very
hard. Familiarize yourself with the excellent inspection
and debugging tools present is every modern desktop browser!