Is there a camera app that I can use as a source for an emulated device running on Genymotion?

Yes, you can use your host webcam as the Android camera source in Genymotion Desktop. You can also inject image or video files as the device camera source with Custom media. See Camera and Media Injection for the webcam and media-injection options.

In Genymotion Desktop, click GRANT ACCESS to expose your host webcam in the camera list. Then assign it to the front or back camera, or choose Custom media and browse a video or image file.

This feature is not available in Genymotion PaaS, and OBS Studio virtual camera does not work on Windows.