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, or use OBS Virtual Camera as the camera source in Genymotion Desktop 3.10.0+; Genymotion Desktop’s media injection widget can also use image or video files as the device camera source. Camera and Media Injection How to use OBS Virtual Camera with Genymotion

For Genymotion Desktop, open the Media Injection widget and select the camera source in Inputs mapping / Video. If you use OBS, set its output to 1280×720 first because Genymotion’s camera API is limited to 720p. How to use OBS Virtual Camera with Genymotion