These are some pages where you can find samples for your projects. Remember that samples can be copyrighted. It is up to you to make sure that you can use the samples royalty-free in your projects.
The Freesound Project is a collaborative database of Creative Commons licensed sounds. Freesound focuses only on sound, not songs. This is what sets freesound apart from other splendid libraries like ccMixter.
Vocal & A-Capella samples - some of the singers request that you contact them first, if you plan on using their voices for commercial projects.