Formula for breastfed baby?

My LO is 4 months, he’s only had breast milk. My husband and I are visiting our families for the holiday and I tried bring some frozen milk from out starch and it didn’t make it 😩 our LO is going to be watched by my in laws over night while we go to a wedding and of course I don’t have enough milk saved up. What’s the best formula to have to supplement with?