Where get the source code for Dovecot package, which is shipped with Plesk?
Warning: modification of the code will be considered as customization, which will not be supported.
Dovecot shipped with Plesk does not have any modifications of source code, thus it can be obtained from official Dovecot site.
Note: it is better to use the same version of Dovecot as shipped in Plesk in order to keep integration. Check Plesk Release Notes for details about packages included into the latest Plesk version.