You could go to incompetech.com, it's got a big library, though I am uncertain if it's really suitable to what you're doing, not much free stuff is likely to be when it comes to action, suspense, and horror...
I primarily use
SmartSound, it's a scoring application that gives you the most control over special music files designed specifically for its use (they have a huge library that just keeps on growing at a very fast rate). The vast majority of cinematic tracks are recorded by real orchestras, and they do deliver true Hollywood quality material, it's amazing! Do check it out, I actually just got several more discs that were released just last month (introductory prices of new albums are insanely cheap, generally $25 per album, then they go up to a normal price of anywhere from $100-200, but there's special sales pretty much every month or so).
I bought the Richard Band vol 2 Horror album a while back an it is awesome! Most of the music sounds like what you hear in our modern horror flicks, and a few sound a bit more oldish, but work for modern stuff too. I also got Richard Band vol 1 Action which has some really good action-packed material; I have Orchestral Impact which has even more action, probably the best yet for my needs (quite a few tracks sound very much like the Bourne Identity trilogy music); Fear and Uncertainty has a few really good action tracks, and some tension-builders; Theatrical Impact is loaded with epic music with the chorus and everything--I totally love it, it's got the perfect tracks for final showdowns and gripping trailers!; Cinema Standards is a variety of tracks, comedy, action, suspense, romance, sci-fi; Cinematic Excellence has great, beautiful tracks for scenic places and events (like scenic castle sequences), some sad, gut-wrenching tracks, some romantic; Sound Palette Weapons and Destruction is a sound effects album loaded with varieties of sounds, different versions of each kind, like 8 or so different pistol shots, explosions, shotguns, automatics, cars driving by, toilets flushing, glass breaking, etc;...there's plenty more I would have, I just need money and wait till the next big $25 per album sale, these guys make incredible quality stuff! I used parts of several tracks in my
Hall of Wrath video, and on the wedding trailer on my
business website.