Supplementing formula?

On the days that I am home with my 14 week old I can exclusively breastfeed with no problems. However, the 3 days a week that I work (I am away for about 14 hours) I can not keep up with his demand. He eats about 19-30 ounces on my work days and I have only been able to pump 9-14 ounces while at work. I have a very small freezer stash built up but as you can imagine, we are flying through that. I have tried lactation blend supplements, fenugreek, lactation cookies, frequent pumping or power pumping... You name it, I've tried it. I'm afraid that I will have to start supplementing baby with formula. I feel so guilty using formula. What are your experiences? What's the best formula to try? Should I wait until my freezer stash is gone or start supplementing with a bottle a day now? HELP please!!!