Supplementing with formula

I have a 6.5 month old who is still exclusively fed pumped milk. The last couple of weeks I have noticed my supply is starting to dwindle. I’m having a hard time keeping up with how much she’s eating. I’ve been having to get into my freezer stash and it’s starting to dwindle as well. We are thinking about supplementing with a little bit of formula, but I have absolutely no idea where to start.. what kind of formula do you recommend? How much do i add to a bottle?