EBF but need something to supplement

Stephanie
My son is 9 months old. I am pregnant again and under a lot of stress. I have been pumping but have not been getting enough for the next day for the last few weeks so that has really depleated my freezer stash. I am thinking I need some formula as a back up in case I continue pumping less. Anyone done this? Suggestions on type of formula? He has never had any so I'm not sure how he will react.