public function getUnique($sku, $store_id)
    {
        $existing = $this->get(array('sku' => $sku, 'store_id' => $store_id));

        if (empty($existing)) {
            return $sku;
        }

        $counter = 1;

        do {
            $modified = $sku . '-' . $counter;
            $counter++;
        } while ($this->get(array('sku' => $modified, 'store_id' => $store_id)));

        return $modified;
    }