You can approach Airlines for co-branding, ask shops in Airport to promote the app, ticket booking sites to promote your app in signature of the mails they send .. there are lot of avenues. What exactly are you trying to build.

Call me if needed. I am consulting architect with 14 years of experience in building iOS, Android, Windows Phone mobile apps, windows and web apps that scale.

Aim for a site like flippa.com which is a marketplace to sell websites and apps.

Let me know if you need more help.

Businesses (therefore also apps) are not sold, they're bought. It will be tricky to find a buyer while still having the upper hand. There are different things to trigger interest but it depends on your specific app.
The best way is to let them know about you and then start talking without writing you want to sell in the first cold email.

There are a couple big companies you can approach to sell your app:
https://www.kingfiloholdings.com/
https://empireflippers.com/
https://flippa.com/