My supplementing nightmare realized...

I'm looking for tips from working mama's who had to supplement with formula to keep their growing baby fed. Should I mix formula with breast milk? Just try different kinds straight up until I find one that will settle in his belly? This is the deal:

My babe is 20 weeks old and has been ebf. I had an impressive freezer stash ready to go for when I went back to work which he has blown through in the last 3 months. I'm nearing the end of the stash and the math isn't adding up. I've got a few weeks tops before my pumped milk and backups aren't going to cut it. I want to just give him enough formula to allow me to get ahead of the game I feel like if he just had a few formula feeds a week while I'm working I could keep up with my supply. Soooo today we tried to give him 4 oz (usually he takes 6-8 of breastmilk) formula and he drank it down fine. Soon after he puked all over. Twice. Hubby fed him a breast milk bottle and he threw THAT up. I've been home since and he's breastfed fine. Never seems fussy or upset just maybe a little gassy. I'm so afraid I'll get to the point where I'll be out of pumped milk and frozen we have to figure something out.