I'm gonna guess your projects are low budget, so your best idea is to look at places that license royalty free music. Doesn't mean you won't pay, it just means you won't have to keep paying over time if your project gets out there and stays out there over a long period of time.
A few places I know right off the top of my head are Music Bakery and Pond5. Music can be anywhere from $10 to hundreds of dollars - and of course, the really good stuff is more expensive. But I also suggest checking local artists like Scott said. They may be thrilled to have their music in your project for little or no money. Just make ABSOLUTELY SURE they get a mention in the credits. It's extremely rude to not give credit to crew and artists, especially if they did it for free.
Best of luck!
The real truth is that this depends on your budget. Due to copyright laws, using music that is popular is often very expensive. This is especially true in film. For an introduction to this check out: https://www.filmindependent.org/blog/the-filmmakers-guide-to-music-licensing/
Now, if you are on a tight budget, getting to know local artists and working with them to use their music in your film can save you money, get the artist exposure, and sometimes lead to something even better than using popular music.