view
-------
$('#source_type').change(function(){
	$.ajax({
		type: 'POST',
		data: 'source_ref'+$(this).val(),
		url: "<?php echo base_url('documents/get_sources_location'); ?>",
		success: function(data) {
			$('#source_location').val(data);
		}
	});
});

controller
------
public function get_sources_location()
{
	$source_type = $this->input->post('source_ref');
	$this->load->model('documents_model');
	$results = $this->documents_model->get_locations();
	echo json_encode($results);
}

model
------
public function get_locations()
{
	$query = $this
		->db
		->select('location')
		->where('classification', $this->input->post('source_ref'))
		->get('sources_location');
	$data = array();
	foreach ($query->result() as $row)
	{
		$data = $row->location;
	}
	return $data;
}