Formula milk brand

Im planning to mix feed my baby with bm and fm. Reason for this is to top-up in case my supply falls short. Then subsequently feed him fm every night before bedtime. My boy just turned 3 months and i recently fed him with bm + lactogen. He drank all but the next day he had constipation. So that's when i know the brand doesnt suit him or he probably doesn't take mix feeding well. But i wanna try again with a different brand. Any recommendations?