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.