I'm not sure that exists, because music will typically belong to the creator and/or performer. So I don't know about that. You might be able to buy a royalty free CD of music or find some online. I did read on another board about someone doing slide shows for their photography businesses and what they did was to have the client pick out and purchase the music they wanted. Then she used that music in the slide show. You'd have to check the legalities of that, but it makes sense because the music was purchased by the client who was using it for his/her personal use. May be legally splitting hairs. If I can find that thread I'll send it on to you pm.
BTW, I found a program that I really like that doesn't cost an arm and a leg. It's called Magix Photostory on CD and DVD. It's about $40. Since it works with WMV/WMA files you can also use it with Windows Movie Maker and Windows Photostory 3. You can get a free trial of the e-version (with fewer features) on their site, but I'd recommend getting the boxed version because it comes with a lot more. Steep learning curve with this one, cause so much is hidden in the interface, but if you can master photoshop this will be a breeze. I can't compare it to the more popular ProShow Gold, because I haven't used it, but it has a lot of transitions, tons even and you can customize some of them to do just what you want. You can blend two photos too which is kind of cool. When I went to buy it (at CompUSA, but Circuit city also has it I believe.) I told the salesman what I was looking for and he raved about it, even as we were standing in front of some other much pricier similar software like Pinnacle. He said he'd tried at least 5 and this was the best by far. Just a thought.
If you want to see a sample of what I did, I'll send you the slide show I just did of my oldest so it can be played in WMV format in Windows Media Player. I'm eager to share this so it would be no problem.