That's a rabbit hole right here. Pay someone famous to say something encouraging to you based on your willingness to pay them to say it rather than anything about you meriting encouragement!

Also, if you could coordinate a few different folks saying various lines & edit those together, you could construct a pretty Fahrenheit 451-type experience.

I've listed a ton of options in the comments of http://www.poetv.com/video.php?vid=169624 , and more recent additions include Flava "Affirmative Male Child" Flav, Ernie Hudson (who used to believe anything you told him if you paid him enough; now he just says it in a video) and the voices of the lovable horrible creatures from before the dawn of reality from Satellite City.

I want Gilbert, of course, but this is magnificent.

Ooh, found Gary Busey.

Also the actresses who played Sabrina and one of her aunts (the responsible one) in the 90's, Tommy Chong, Ice T, the voices of Duke Nukem, Gumball and the Cryptkeeper, the Unknown Comic, and Mr. Bubz, the Dog Who Hates.

Oh, and Michael "MAAAAA" Rapaport.

And Joey Fatone from N*SYNC, the actress who played Frau Farbissina in Austin Powers, and Uri 'Fuck Kadabra' Geller.