If you are stuck, here's some sampling idea's
Sample a sound (e.g. Kick drum hit) and use it in a sequencer as percussion. I heard a tune by Sabaton that used a gun shot as a snare and a bomb explosion as a kick drum...I kid you not!
Can't sing? Do what I do, get a vocal from somewhere. James Brown, Bruce Lee, Darth Vader and Christopher Walken have all 'contributed' to my past creations...
Sample a loop of a song you dig (bass loops are a great place to start) and work off that. Usually easier to find stems (e'g' raw tracks of each instrument) than extracting something for a song.
Work with stems - sometimes artists release stems for peeps to remix.
Here's a list of albums/songs that officially shipped with stems (Dream Theatre, NIN et al), not a long list though
Here's a list of 5 places to find stems - again not a huge list but it's a start!
And finally, a perhaps not official site with a lot of stems...
Happy sampling peeps!