[TYPO3-mvc] initializeAction how to instance repository from other extension?

[ralph]

hi list. as i am still learning how to deal with mvc i struggle with 
instancing a repository from another extbase extension.
i want to use a foreign db table in my controller. the foreign one is 
already present in another extension. i tried so far:
/**
* @var Tx_My_Domain_Model_SomeRepository
*/
  protected $someRepository;

and then

  public function initializeAction() {
	other instances from own plugin
	$this->someRepository = 			 
t3lib_div::makeInstance('Tx_My_Domain_Model_SomeRepository');
}
any hints how to make the foreign repository accessible?