Flyleaf songs aren't normally played on the cello, so I doubt there is sheet music or anything published. All I can is, try to hear it out, and write it down. Then you can work on it until it meets...
Any classical music is suited to cello. Popular music is probably better for a guitar or a piano, but anything by Brahms, Bach, or Mozart will sound lovely on cello. I like Minuets and Etudes, but I...