Google for it! Seems obvious. However, there is an overwhelming amount of stock music out there and the quality, price, and usability of many of websites vary widely. Maybe I'm not so good with the Google-fu but it took me several months to write down a good list of sites that offer what I need at a reasonable price.
The first time I became actually aware of great stock music and where to get it from was through Dave Dugdale's Youtube channel. I found the track "Hummer" great for the energetic videos I was producing and consequently put it in the first half of my TWALK video (watch below). Premiumbeat is one of the more pricey sites out there but I their content is all high quality so there's no weeding through a lot of so-so music. Production value on all tracks is top notch. The big advantage from a licensing point of view is that you can reuse the music tracks in as many free and commercial pieces of work as you want. When buying a track the user can choose between the full song, loops or individual samples.
Audiojungle takes the second place in my ranking. It's got huge variety and the prices are much lower than on Premiumbeat at $15-20 per track or around $30 for music packs. Because Premiumbeat is curated it feels like it's got its own sound whereas Audiojungle can be a bit different and refreshing. However, one big downside of Audiojungle is that a track or music pack can only by used in one (free or commercial) piece of work. I'm not an expert and this is definitely not legal advice but it seems like a song can still be used in multiple videos as long as they belong to the same project, so for example the main video, remixed in a trailer, and maybe an alternative cut or follow up video or making of/behind the scenes. In this case, $15-20 for 2-3 videos isn't bad value.
So if you intend to use a track only one time, Audiojungle will be cheaper. If, on the other hand, you want to use the same track in the future, the $40 (edit: I wrongly said that a song costs $60 in a previous version of this article) of a Premiumbeat track can quickly pay for themselves.
Vimeo also has got a stock music store. While the prices are very low (one or a few dollars per track), unfortunately the quality is much lower well and so far I haven't found anything that I wanted to use in my videos. I'm sure there are great songs and something on Vimeo might work much better for a particular video than another song from Audiojungle or Premiumbeat but if it's going to take hours to find, it's not worth the saved money. If you like to sort through a lot of tracks this is worth a look. Another nice feature is the close integration with the Vimeo video site.
On the free side of things there are sites like the Free Music Archive, Incompetech and Musopen. Many tracks on these sites sound very different from your usual stock music so it's definitely worth a look but again, I find it hard to impossible to find the right kind of music with good production value for a trailer or a short video. I could imagine that they are very good for background music on narratives especially since you can probably find about any kind of music on those sites.
Also check out lists of useful resources on reddit and one of the many, many videography blogs out there. There are a lot more music and sound repositories, both free and paid, that I haven't mentioned yet.
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