Where to put plain object classes?

Rufnex is correct, just as a side note thou:
You would of course use $this->load->library('PlainObject'); to load this class and of course this would automatically attach it to the CI super object as a singleton. $this->plainobject->method(); etc...

If you need to create multiple "plain objects". You can then always $foo = new plainobject(); after calling $this->load->library('PlainObject');
because of course $this->load->library('PlainObject'); already loaded the class.
You of course have the singleton still attached to the super object thou.

Thank you all! There's an alternative way to instantiate multiple objects without calling first $this->load->library('PlainObject') ?