FTM and First time giving formula

My LO is 9 months. She was exclusively breastfeeding. Yesterday at her Drs appointment we found out that she is not gaining weight, so we need to start formula. I'm clueless about how to transport formula. Can I prepare it at home or I have to bring the powder and water separate?. I saw on amazon a bottle that has the powder on the bottom and water on top, so you mix it when you need to use it. I wonder if this item is a must have or useless. What other things I should have besides bottles. TIA!