I found these:
<a href='http://ftp.riken.jp/Linux/slackware-iso/slackware-4.0-iso/slackware-3.9-install.iso' target='_blank'>http://ftp.riken.jp/Linux/slackware-iso/sl...3.9-install.iso</a> 28-Sep-2005 10:49 648M
<a href='http://ftp.riken.jp/Linux/slackware-iso/slackware-4.0-iso/slackware-3.9-install.iso.gz.md5' target='_blank'>http://ftp.riken.jp/Linux/slackware-iso/sl...tall.iso.gz.md5</a> 28-Sep-2005 10:49 60
The original Slackware 3.9 CD set was 4 CDs... I guess the next question is: what were you after - just packages, or the complete source for everything?...
Another site is <a href='ftp://ftp.slackware.no/pub/linux/slackware/slackware-3.9/' target='_blank'>ftp://ftp.slackware.no/pub/linux/slackware/slackware-3.9/</a> but they don't have any ISOs for 3.9, just the files that were on the CDs, all together in folders (a1, a2, a3, ... xv1, y1). If you're looking to pick and choose from just a few extra packages/source, that may be your best bet. Other non-ISO mirrors are:
<a href='http://search.belnet.be/mirror/ftp.slackware.com/slackware-3.9/' target='_blank'>http://search.belnet.be/mirror/ftp.slackwa.../slackware-3.9/</a>
<a href='ftp://ftp.unina.it/pub/linux/distributions/slackware/slackware-3.9/' target='_blank'>ftp://ftp.unina.it/pub/linux/distribution.../slackware-3.9/</a>
<a href='http://ftp.be.debian.org/mirror/ftp.slackware.com/slackware-3.9/' target='_blank'>http://ftp.be.debian.org/mirror/ftp.slackw.../slackware-3.9/</a>
<a href='http://mirrors.evrocom.net/slackware/slackware-3.9/' target='_blank'>http://mirrors.evrocom.net/slackware/slackware-3.9/</a>
HTH!