OpenMPT is telling me to search two plugins online, but they're unavaliable. And now?
Here's a backup of those plugins: https://sagamusix.de/plugin/classic_series/
Alright, but where should I put the .dll file of the plugin?
You can put them anywhere you want* and then add them through the Plugin Manager (https://wiki.openmpt.org/Manual:_Plugin_Manager).
*for plugins that don't come with an installer, it is recommended to use a central folder in some place where you have write access to and that you can easily locate.