Pay Per Download Niche Apps?

Hello I had a question on pay per download affiliate programs. So i have a post on a few mobile apps in my niche.Example let's say i have a gun website. I make a post about gun mobile apps. I've read about pay per download affiliate however i'm just confused can i use any apps for this since i only want to promote gun related mobile apps. Do i need to go to a affiliate program that is represented in my specific niche or how do i know if the app i want to promote is in the affiliate programs? Thanks for any helps!I've searched on the internet but nothing is really clear on this.