x264-0.0.0-0.6.20080510 contains libx264.so.59
ftp://ftp.videolan.org/pub/videolan/x264/snapshots/
In fact it is very easy to find : search libx264.so.59 rpm
(rpm packages can show files inside the package by a simple command)
rpm.pbone.net will show the package version :
http://rpm.pbone.net/index.php3/stat.../libx264.so.59
Then search ..(google again) .. x264 .. and find "videolan.org"
To see files in a package : 'lesspipe.sh <package>'
(.tgz, .rpm, .deb .. and may be other suffix's too)
'less' can be used too.
May be there is a Slackware package x264-0.0.0-0.6.20080510 too ??
Rgds