Hello, I'm Beater, and I've encountered the question "How do you find music for your videos?" enough times that I've decided to make a small guide on how you yourself relatively easily can discover some amazing tracks and expand your music library. Don't worry, it's pretty easy!
Oh, and just to get the obvious disclaimer out of the way: There is no definitive place to go to find good music, since everybody have their own unique taste in music. This guide will merely provide the framework on how to go about finding music [i]in general[/i]

[b]Youtube[/b]
First step: Get a youtube account (seriously, get a youtube account)

There are a ton of channels on youtube which are dedicated to uploading music in a specific genre, so by simply subscribing to these channels you'll have a ton of good music in your youtube-feed. Make a habit of checking your "my subscriptions" for music a couple of times every week, maybe make a playlist out of them and listen to them while you do something else and you're bound to stumble upon some stuff you love sooner or later.
It's also worth checking out the related videos and other tracks by artists who've made a song which you like.

[u]Some examples of channels which I like[/u]
[url=https://www.youtube.com/user/Liquicity]Liquicity[/url]
[url=https://www.youtube.com/user/FunkyyPanda]FunkyyPanda[/url]
[url=https://www.youtube.com/user/PandoraMuslc]Proximity[/url]
[url=https://www.youtube.com/user/MrSuicideSheep]MrSuicideSheep[/url]

And there are many more. It's simply a matter of finding channels with the type of music which you like. Use the search function!

[b][url=http://www.soundcloud.com]SoundCloud[/url][/b]
Get a soundcloud account (yes, get a soundcloud account)

Soundcloud is awesome since it seems a lot more 'direct' than youtube. Instead of subscribing to an aggregate account which uploads music from many artists in the same genre, soundcloud tends to be the artists themselves publishing their own music. Usually when an artist releases a new track or an album it will be available for listening on soundcloud even before it becomes available in the shops, so soundcloud is a great way to keep tabs on your favorite artists.

After you've made your account, you're able to search for and follow different artists and if you find a track which you really like you can save it to your favorites or make a playlist containing as many songs as you like.

[i]Some examples of users which I like and follow[/i]
[url=https://soundcloud.com/gramatik]Gramatik[/url]
[url=https://soundcloud.com/grum]Grum[/url]
[url=https://soundcloud.com/matrixandfuturebound]Matrix & Futurebound[/url]
[url=https://soundcloud.com/chromeo]Chromeo[/url]
[url=https://soundcloud.com/chomstars]Chomstars[/url]

[b][url=http://beatport.com]Beatport[/url][/b]
Electronic music portal/shop. You don't need an account to browse, but you can follow artists and buy music with an account, although I don't think the follow feature is worth it.
Thanks to a magical Chrome plugin called [url=https://streamus.com/]Streamus[/url] you can listen to songs in their entirety, and not just the 30 second preview.

Install streamus and browse your favorite categories (you can make a playlist of the top 10 or top 100 in a given category by pressing a single button)

[b][url=http://reddit.com/r/music]Reddit[/url][/b]
Make a reddit account (it's worth it)

They say there is a subreddit for [url=http://www.reddit.com/r/petsgonewild]everything[/url], and music is no different. The main music subreddit is called /r/music, but it's mostly filled with old pop music that you've heard a million times before, so you want to branch out and subscribe to the more specific subreddits. There are plenty of links available in the sidebar to get you started

[u]Some subreddits I like and subscribe to[/u]
[url=http://www.reddit.com/r/NuDisco]/r/nudisco[/url]
[url=http://www.reddit.com/r/electrohouse]/r/Electrohouse[/url]
[url=http://www.reddit.com/r/trance/]/r/Trance[/url]
[url=http://www.reddit.com/r/FrenchHouse/]/r/frenchhouse[/url]
[url=http://www.reddit.com/r/listentothis/]/r/listentothis[/url]

[b][url=http://newgrounds.com]Newgrounds[/url][/b]
Get a Newgrounds account (Ok, maybe you shouldn't get a newgrounds account)

[url=http://www.newgrounds.com/audio]Newgrounds[/url] is a website that was originally made to give people a place to share self-made music which people could use in games and videos without having to worry about copyright claims, which means that any music you find on this site can be used without worrying about copyright infringement, as long as you give credit, of course. The site is a far cry from its former glory, but I felt like I should mention it anyway. You can follow artists and save songs to favorites and the site is really well designed, so it has that going for it, but these days there aren't a lot of people who upload songs regularly to the site.

[u]Some of the artists I follow[/u]
[url=http://f-777.newgrounds.com]F-777[/url]
[url=http://waterflame.newgrounds.com]WaterFlame[/url]
[url=http://geoplex.newgrounds.com/]Geoplex[/url]

[b]In closing[/b]
Finding new, awesome music involves listening to a lot of mediocre, boring or straight up terrible music, but such is life. Listening to online radio stations can also be a good way to find new music, since the playlist is usually publicly available and very easy to find (Trance.fm and spotify radio stations being examples).

Good luck and happy listening