You can find the last 16-bit version of mIRC, 5.91, at
www.mirc.com/get.html. Lots of old mIRCs can be found at
www.mirc.net/mirc.php and there may well be various other archives on the web. You might try searching
Google, although the aforementioned archive has nearly every version you would want (or could even run).
Note, I do not recommend using an old version of mIRC. Newer versions contain bug fixes, new features and so forth. If you use an older version of mIRC, we cannot promise you that we can support you if you have an issue, and many #mIRC channels on IRC will not help you either. You may also encounter scripting errors with the latest scripts and various other bugs.
Regards,