I actually find the quality of packages on linuxpackages.net to be variable at best. I personally find http://www.slacky.eu
to be much better. I don't know if they have the package you want. You can enable slacky.eu in gslapt. That site does have dependency checking and linuxpackages.net does not. For VL 5.9 you want the Slackware 12.0 repository, not 12.1. Just be sure to disable slacky.eu in gslapt when you're done because you don't want to "upgrade" with non-VL packages. The results can be disastrous.
The other option, and honestly the better one if you're willing to do a little more work, is to download the source code and use vpackager. vpackger is a GUI tool that turns source tarballs into Vetcor Linux packages. It does take a moderate amount of knowledge if everything doesn't go 100% smoothly. If the build process is really standard then it might just be very easy to produce a VL package with proper dependency checking.