1) You can make them yourself
2) You can ask a friend to make them for you for free
3) You cam browse for free to use assets here and in the internet, just make sure you know what their terms of usage are
I just successfully tested changing music by region, and changing it between continents on the world map after getting on/off the airship. I had to override the default audio transitions with eventing to make it change smoothly but it worked.