Slackware 7.1 is the primary source of add-ons for BasicLinux 2.x. Packages can be downloaded from a Slackware site. All packages from Slackware 7.1 should work, if you have installed the necessary dependencies (to see what dependencies are required, use ldd). It might also be possible to use packages from Slackware 7.0, 8.0, 8.1 and 9.0 (but the dependencies may be more complicated). All Slackware packages are installed using installpkg or pkgtool.
Additional add-ons for BasicLinux 2.x are available here. They are not Slackware-type packages. Do not use installpkg or pkgtool to unpack them.
You can also find BasicLinux add-ons at James' site and at JP's site.
DeliLinux is based on Slackware 7.1. It has many add-on packages -- most of them will work with BasicLinux 2.x.
The following sites provide packages for Slackware 8 and 9. Some of these will work with BasicLinux, but it may be necessary to install a library from a later version of Slackware.