These independent sources will license music on a royalty-free basis:

http://magnatune.com/info/licensing https://pro.jamendo.com/stock-music-licensing

Musopen claims to host all recordings that are in the public domain, but many are posted anonymously or semi-anonymously and I'd be leery of using them without doing some legwork first. (It's all classical music, which probably isn't what you want, anyway.)

https://musopen.org/

My team and I run a boutique production studio in NYC and we love composing music to visuals. Shoot me an email at "josh at soundassociation dot com" and let's discuss your project!

http://www.soundassociation.com/work showcases some audio scores we created.

I really enjoyed the subtle humor of how hiring an individual sound engineer/musician to actually compose something wasn't even considered an option by the responses thus far.

It's definitely an option.

Moby has a site.. though it appears to be for film, you could ask about a video game. http://www.mobygratis.com/

https://www.marmosetmusic.com/ has some great tracks.

You should contact my friend: http://octophonix.com/

I'm a composer, shoot me an email with some more info and we'll see if we can work something out!

melodyloops.com has some good quality stuff.