Where Can I Find Music for My Mod?

A Word on Picking and Using Music (by Valentin Cognito)
Music is, in my opinion, a very important part of a mod. Depending on the soundtrack you use, you can completely change the atmosphere. I would therefore encourage dedicating time to choosing appropriate soundtracks. Make sure you pick carefully: going for a low-quality or overly short soundtrack will just make it an earsore, and will instead encourage players to disable music altogether!

Obviously, going for licensed soundtracks is not recommended (as much as you might want to show Lillian approaching you to the sound of One-Winged Angel). Fortunately, the Internet has no shortage of royalty-free tunes, and with a bit of time and effort, you can dig up some gems! Just don't forget to properly credit the music you're using (typically within a Read Me file)!

With that said, be careful not to overburden your mod with an overabundance of soundtracks. Not only will it quickly make your mod heavy to download, but it'll also get in the way of a good atmosphere. Be discerning, and remember that going for recurring themes is good to set the ambiance! And of course, don't hesitate to pick from MGD's existing soundtracks, which you can find in the  folder.

And with that out of the way, here are the most popular sources among the MGD modding community for royalty-free music, sorted by relevance. They are used by Threshold himself, in addition to many modders.

Websites
Purple Planet. Probably the most used. Quality music sorted by categories. Probably lacks a bit of a "video gamey" feel, but still very solid.

Wingless Seraph. Good material, very video gamey, sorted by categories. Good stuff.

MusMus. Japanese websites, but featuring classics you may have already heard in other games like this one or that one.

Dova Syndrome. A Japanese website. More of a wild shot, since it's all moonrunes, but there's some good tunes there.

Peritune. Japanese website with a gallery of free music. Titles are pretty explicit, but there's no categories, so some digging is going to be required.

MaouDamashii. Includes a "GAME BGM" category with nice, retro-ey video gamey music. Might be what you're looking for, provided you're willing to do a bit of digging, particularly if your mod features a very "classic video game" type adventure.