To all breastfeeding mommies who have supplemented!!

I am the maid of honor for my moms' wedding next month. I have a two month old whom I will be leaving twice overnight in September. I had a small stash put away for these occasions, but things happen... so now it's gone. I will be trying to pump until then; but I doubt it will be enough. Question!! What formula have you been able to supplement with resulting in the least amount of problems? (spitting up, constipation etc.) and should I start introducing it now?? TIA♡