I don't know where to find sheet music for it, but I think I can help you anyway.
Look for some AIWIY video's (I recommend Ralqh) to listen what it sounds like to play.
What he plays is basically Ab - Db for the verses (Or A - D, whatever key you're playing in)
Fm - Db for the prechorus ("All the promisses.....")
For the part after "When All I Want Is You...." hit Ab and Db, but play around with the surrounding keys to get the actual 'riff'.
I hope you get what I mean a bit, I might record a video to show you what I'm talking about.