http://www.stockmusic.net is an option, but I would probably recommend to do an express production in GarageBand using a couple of loops. Using loops doesnt require you to have that much musical knowledge.
I like jewelbeat.com. The other one I use is the free trial of videoblocks.com. They give you a free 7 day trial and you can download 20 or so videos/music a day. I've done that a couple times Just make sure you cancel if you don't want the subscription.
Freeplay music is another choice.
They charge $0.99 for personal use music (except for use on Youtube, because they put their ads on your work).
Their music covers the spectrum from classical to world to electronic.
i use premiumbeat.com because they offer 3 formats for each song (full length, :15 :30 and :60 bundle, and loops bundle) so if you find a song you like, you can get whatever format will work with your length. plus, i find their license prices to be very reasonable compared to other sites. i believe every song is $40 for an individual format, or $70 for the combination of all 3 formats. many other sites price each song differently based on popularity (meaning the good songs are always more expensive) and I've seen licenses go for $70 all the way to $500 elsewhere.
also there's a lot of good music available via a "creative commons attribution" license (you can use it for free, even for commercial projects, as long as you give credit to the artist ), but it's a pita searching for it. for example, there's a lot on jamendo.com, but they also try to sell you "professional" licenses (even though you don't need it for the cc-by stuff on their site, and there's also a lot of music licensed under different terms (lice cc-sa). also there's a lot of bad stuff as well.
remember to also contribute something to the cc/free - scene if you're using that stuff on a regular basis.