First, you should download the suitable version for your system from here: https://pecl.php.net/package/mcrypt/1.0.3/windows
Then, you should copy php_mcrypt.dll to ../xampp/php/ext/ and enable the extension by adding extension=mcrypt to your xampp/php/php.ini file.
The recent versions of XAMPP for Windows runs PHP 7.x which are NOT compatible with mbcrypt. If you have a package like Laravel that requires mbcrypt, you will need to install an older version of XAMPP. OR, you can run XAMPP with multiple versions of PHP by downloading a PHP package from Windows.PHP.net, installing it in your XAMPP folder, and configuring php.ini and httpd.conf to use the correct version of PHP for your site.
Right from the PHP Docs: PHP 5.3 Windows binaries uses the static version of the MCrypt library, no DLL are needed.
http://php.net/manual/en/mcrypt.requirements.php
But if you really want to download it, just go to the mcrypt sourceforge page
Source: Stackoverflow.com