I am surprised that you did not find it at Whole foods because that is where I bought mine. I would call around to different locations of WF because I am sure they have it. The better and more pure it is the better it tastes.
I for some reason have two bottles of peppermint extract, it's the McCormick version but it's most definitely peppermint extract, not flavoring. Any grocery store should have it, I got mine at Stop and Shop