I know this is tipping my Weeaboo hand a little, but the OSTs from anime are often really good sources of music. I've gotten a ton of mileage from my Code Geass OST, and there's a lot of other ones with great soundtracks. Just avoid the ones with words, or which get a little too J-Pop-y.

Video Game music is also good, ideally the more modern and less easily recognizable ones, because they usually are made to loop decently and have good ambiance.

Other than that, troll You-Tube* for instrumental music. The same goes for "Trailer Music;" basically anything that makes it into an 'Epic Music Mix.'


*Do not be a troll on You-Tube; that's like pissing into an ocean of piss.