Specifically modules, because of two reasons:
- They doesn't sound like **** (like MIDI does)
- They're still small unlike even compressed PCM audio such as OGG/MP3.
Could try modarchive, but the legal status of many modules there is unclear. Normally, if it was to be a free game this wouldn't be a problem, however it is for a commercial product so I need something on permissible CC license (without "nd" part) or public domain. And no, unfortunately I don't have money to hire a composer.