Most of the mirrors have this notice:
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If you're reading this on ftp.slackware.com, you will find that the
ISO images here cannot be downloaded. This is because we simply don't
have the bandwidth at ftp.slackware.com to provide ISO images. ISO
downloads take a long time, and block people from retrieving small
updates.
If you're looking to download the ISO, consider using BitTorrent.
If there are active torrents available, information may be found on
on
http://slackware.com. BitTorrent is available as a package for
Slackware in the /extra directory.
Mirror sites may also have the ISO images up for FTP.
Use the .asc files here to verify your ISO images are authentic.
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That said, you can sometimes find a mirror that hosts the complete iso's (academic sites, for their students, but you can sometimes download from there as well), but they change from time to time, so it's hard to recommend one. There's one in austria that I used recently, but I'm hesitant to post a link, because I'd hate for them to get swamped and remove it, but if you're interested, you'll find it.
Better yet, try out BitTorrent, and save some bandwidth.
