I believe <tt>ksh</tt> is now Open Source (as is <tt>pdksh</tt>), so you should be able to download and recompile it.
However, have you tried <tt>zsh</tt>? It's preinstalled, and does pretty much everything <tt>ksh</tt> does along with lots more really useful stuff. It's worth it just for the amazing filename completion (I haven't used <tt>find</tt> since I switched!), but it also does intelligent prompts, spelling correction, and loads more. It will even emulate <tt>ksh</tt>, <tt>csh</tt> or <tt>sh</tt> for you! I was a <tt>ksh</tt> user, but when it wasn't supplied with OS X I tried <tt>zsh</tt> just to see, and now I use it at work, too. Try it.
(BTW, I've no connection other than as a happy user