Growth Spurt and Supplementing HELP

LO is 27 days old today and has been going through a growth spurt this week. I apparently have flat nipples and have been using the shield and also pumping. I am enjoying pumping because it hurts less and also other people can feed him which offers a nice break. I have gotten used to only pumping and now he's going through a growth spurt and I can't keep up! In the beginning I was producing more milk than what he was eating so I was able to freeze some...thank goodness! We have been using some of that and then freshly expressed milk in the bottle too. When I breastfeed I feel like he's not getting enough and now with the growth spurt I really feel like I can't keep up. I am thinking I have to start supplementing but am terrified because of all of the crap they put in formula and organic options are SO expensive and I really want to be able to provide for my baby!! Is this just a phase and once the growth spurt subsides all will be well or is his hunger just going to continue and be too much for pumping? Any help or advice or peace of mind would be greatly appreciated!!