Uh, this is essential. At least for me. This makes me want to write and to evade writer's block which is non-existent most of the time for me.

Where can you find them?

You can find them in Pixabay as well as Pexels or YouTube, with the latter being my go-to option.

There are mobile games with stories like Choices, Episode and Romance club with many good background songs. You can even use movie themes for this. Mostly I use the following YouTube channels to download the songs and use them while writing each incident.

Romance club background themes (the game is available on Play store, amazing if you ask me) -https://www.youtube.com/channel/UCbld6nZEwpYYczrH9QuCaJg

Choices background themes - https://www.youtube.com/results?search_query=poki+chu

Episode background themes -https://www.youtube.com/channel/UCzHseZz1hHeH7Y-xjRGvafA

Other background themes channels (Credits to the creators of the channels):

https://www.youtube.com/c/BrunuhVille

https://www.youtube.com/c/thesecession

https://www.youtube.com/user/WaterTowerWB

https://www.youtube.com/channel/UCV-OE1O2SpfSaneY5awNR0w

https://www.youtube.com/channel/UCml6b9K7cqayPAthrxMVXlw

https://www.youtube.com/c/RoyaltyFreeMusicNoCopyrightMusic

https://www.youtube.com/c/RossBugden

https://www.youtube.com/c/EpicMusicWorldII

https://www.youtube.com/c/NoCopyrightSounds

You can't use these songs like, in projects that are public without permission. Ex - making a trailer for your book and posting it on YouTube. However, you can use it in the multimedia platform of Wattpad with credits.

Credits go for all the YouTube channel creators above mentioned. 

Links are mentioned in the comments by me.