I'm not sure which package you are actually missing
. But take a look into Slackware's homepage. They've got plenty of mirrors with older Slackware's releases. Most likely you will find the package you seek in there.
It's also possible to convert an rpm to a Slackware native package (.tgz) by using rpm2tgz or Alien. But I don't think that should be necessary since Slackware 8.1 is supported... it's just a matter to figure the package you are missing...
Sorry that I can not offer more help
Regards!