I sometimes listen to tracks from musicForProgramming[0] website, and I usually download the ones which I like and then play them off the offline music player.
Other than that, I have some Spotify playlists saved (not from me, some links I found on Twitter and other sites) [1,2]
Depends on the user base as well. Which users are likely to use the app and you're able to retain easily.
Another factor is how well you / developer knows the platform. For example, if you have always been writing React Native apps, then you can do RN but focus only either iOS / Android app. That way you can give one platform your 100% and then quickly launch to the other platform with lesser work.