http://www.subsonic.org/pages/download.jsp has a download for that. Keep in mind that Subsonic is basically abandonware at this point, so support will be limited to community and there will probably not be any fixes/updates.
You might look at a lateral move to Airsonic Advanced https://github.com/airsonic-advanced/airsonic-advanced - It is in active development with frequent snapshot updates https://github.com/airsonic-advanced/airsonic-advanced/releases . Same feature set as Subsonic (API, Sonos, etc.) with updated code. As it is open source, you also get all the features Subsonic Premium gives you, but for free. You need your own DDNS solution for it, but it it can be run on Synology via Docker (similar to https://www.youtube.com/watch?v=EXHpKl_LVJI&ab_channel=boredCaveman)

