I a think that Google Adwords is the best way to promote an app like this. But if you need something cheaper, then social media would be a great choice. You can run a Facebook campaign or Instagram campaign.
Also, you can try to promote your app through some local (US based) online journals or blogs.
And you can try to engage YouTube or Instagram bloggers. Ask them for an advertisement on their accounts. It's not a cheap campaign, but it would be really effective.