If you have a certain type of music you like (hip hop, rock, ect) you could try to find smaller or local artists (or even DJs or producers) that make music you like. You could ask them to use some of their music for your videos, with the promise of clearly crediting them. Promotion for promotion, one hand washing the other. Or you could find royalty free music or join a pay site that has lots of music that your membership licenses you to.
Or like meggisonj said, make your own. FL studio has a demo version that has everything (all vsts and plugins) free that you can use. You just can't save your projects. But if you watch tutorials and just make the whole track at once you can bounce it out to a wav or mp3 and have that to use. Good luck
