In a regular Drupal module, one would usually use the plugin architecture, but that comes with its overhead of boilerplate code and may not be the solution for the simplest of use cases.
Many class finder libraries rely on get_declared_classes() which may not be helpful, as the classes in question may not have been declared yet.
If you are on a Drupal 8/9 installation and want to use components already available to you, the Symfony (file) Finder can be an alternative for finding classes in a given namespace.
Ouside of Drupal 8/9, you may need to require this library in your application:
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