If you’re into photo wallpapers (as opposed to abstract or 3d rendered ones for example) Unsplash is great.

It’s a website, but they also have an app. It’s a completely free stock photo website, except the photos don’t suck like stock photos usually do lol

ArtStation is another good one if you’re into more paintings and renders as wallpapers. It’s an app for artists to share their work, but you can just save/screenshot the images you like and use them as wallpapers.

As an artist myself, I promise you 99% of artists don’t care if you do that as long as it’s just for your personal use.