I've had the same issue, and finally decided to load a model within a model, when it is needed. This is not against MVC, because MVC is more about data flow, than system components including (loading). 'Normally' (without CI) you would use class autoloading (http://www.php.net/manual/en/language.oop5.autoload.php), while Codeigniter doesn't support this loading models within models is not a sin.
It is also good to notice, that even if you'll load some model more than once, Codeigniter will just ignore it. So even if you load some model twice, nothing bad will happen.