You can use "App Name Check" for free here

Try App Name Check with different words or names. Curious about even more original features? Then go to the awesome NameRobot Naming Toolbox, where there is App Name Check with all the trimmings. There are even more name generators and clever tools for word search, naming, and inspiration.

Is your name already being used for an app?

The App Name Check lets you find out how often your name idea appears in the various App Stores.

The check searches the most popular app stores such as Google Play Store, iTunes Store and Microsoft Store, but also includes other platforms such as Amazon.com apps and even Firefox addons.

Particularly useful for all those who want to do something directly with apps, but also for related areas such as games, software, blogs and everything online/mobile.

You can select and deselect App Stores in the Settings.