Anyone feeding breast milk and supplementing formula for low production?

Carina • 25 years young. baby #1 born April 11th 2017❤️
My son will be 4 weeks old on Tuesday and I have been exclusively pumping breastmilk since 2 weeks old. At first, my pumping production was pretty good. My morning pump would get out 10oz and then every 3-4hours I would pump 6-7oz more. Last week I went on a day trip and didn't pump for 12 hours because I had nowhere to pump.. The next day my production dropped to only a few oz. I drank mothers milk tea and got it to come back the next day to only about half of the production I was making prior. Baby is hungry and I am not producing enough as fast to keep him full. I will need to supplement formula for a couple of feedings for the next few days but I don't know how? Any other mommas giving both breastmilk and formula? How do you switch it out to make sure baby is healthy and happy? Thank you😊