supplementing

I've been EBF my daughter since birth. She is 4 1/2 months old. I recently went back to work and am able to pump 4 times a day there. About 1 month post-partum, I developed mastitis that turned into an infected abscess. It was surgically cut open and I had drain put in and removed 11 days later. All this trauma left the breast yielding very, very little. I can pump for 30 minutes and I get just drips.  I have tried many ways to improve my supply but the damage to the breast is already done. 

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I've been pumping 4-6 ounces total a day at work while my daughter drinks 12-14 ounces at the same time. My freezer stash is rapidly disappearing. My one breast cannot keep up with the demand and I feel I have no choice but to supplement. 

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I plan to continue to pump 4 times a day at work and nurse while I'm home. 

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Since I was so heart set on breastfeeding I did no research on formula. What are recommendations for formula used as supplement? Daughter has never had problems with gas or reflux. 

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Thanks for your advice!