https://apps.obtainium.imranr.dev/ has plenty of apps that don't have F-Droid releases. Many of them come from the developers own website.

And if you see anything is missing, you can always add it!

The other way is to use fdroid as source within obtainium. The benefit is to maintain app list at one place, so you can export/import to another device, when needed.

You can use it with non-github releases too, but it's super rare when one isn't available. In the case when Obtanium doesn't work, I just use F-Droid.