Breastfeeding and surgery. What to do?

I’m having emergency gallbladder surgery and my daughter is exclusively breastfed. I didn’t have any milk stocked up as i had no idea this would be happening. She’s never had formula but it’s looking like i am going to have to supplement for a day, what formula would be easiest on her tummy? I feel absolutely awful. I would call her pediatrician But they’re closed on sundays :(.