This is offered in the free sampler app for the AmbiScience Ultimate Sampler App for iPhone. There is an ad at the bottom that comes up for American Express Open card. When I clicked the ad, I was given a $ 5 iTunes card.

I’ve had the app for a few years and really like it and bought paid versions that I use to help with focus while working.

I think this is the link:
Brain Power | AmbiScience Binaural & Isochronic Ambient Mind Utility by Tesla Audio Sciences
https://itunes.apple.com/us/app/brain-power-ambiscience-binaural/id300108404?mt=8

Edit: Sorry for the vague info. I just realized that link is not the free version. I have several of these apps that all look alike. There is a free one named with “300 lite” but that one does not seem to have ads.