I went over the Apple's iTunes store to find David Archuleta's version of John Lennon's Imagine from last night.
Wasn't there.
Found it on Google though: David Archuleta — “Imagine” — American Idol 7 Top 20 — Video MP3 : rickey.org — American Idol 24/7
Update:
Following the FOX broadcast, you can pre-order your favorites and you will receive an email as soon as the performance is available for download. Music performances will be available for $0.99 per song.
I signed up to get an email update when the song is available.