Switching to formula

Alyssa • Mom of 1 beautiful baby boy 👶🏻 7/8/19

My son is 4 1/2 months old. I exclusively pumped since he was born and stopped pumping when I went back to work. I have about a month left of frozen breast milk to give to him but then I am going to have to switch him over to formula. Any recommendations out there? Also has anyone tried the Costco brand formula?