Pumping while baby is away

How do you ladies keep your milk up while away from your baby? I pump every two to three hours, and eat lactation cookie, but my supply seems to be dropping. My son eats about 3-4 ounce every 3 hours. He's currently staying at my mom's house due to part of our floor falling through. And our water pipe busting. So I pump and send the milk to my mom's. But I feel like I'm not producing enough to get him full. I did give my mom permission to formula feed if she runs out before I bring the next batch over. But what do you do to keep your supply up?