You could fetch the source from
the GNU Readline homepage, but if you intend to edit anything, you should probably go with the source that comes with your distribution.
You run Slackware, no? In that case, recompiling readline shouldn't be a problem. Get the sources from source/l/readline/ in the Slackware distribution, and compile using the included SlackBuild script. This ensures you have it compiled with the same options, which preserves system reliability.
Cheers,
Odie