There's a lot of music out in the world that is "royalty free", which can be used as you suggest. Of course, you may have to hunt to find the right kind of sound for your project. Most of this music still has to be paid for (one time, up front, usually), but there are a few artists that offer their work for free, either as a sampler or just because they want to share. Also, some of the places that sell royalty free music occasionally give away samples.

Check out www.istockaudio.com to see if they have any samples, and/or do a web search for "royalty free music".

Oh, when you do find some, check the terms, because they can vary (some allow commercial use, some don't, almost all restrict redistributing the music).

If you plan to do a lot of animation and you're so inclined, you might take a look at Acid by Sony to make your own. They have an introductory version as well as a more sophisticated (and expensive) version. Acid lets you make your own music from sound loops, and there are a lot of free for use sound loops out on the 'net, too.

Hope some of that's helpful.