I thought I would walk through the process that led me to the song, which, was not part of the Whip It soundtrack or listed on the movie's Web site or Wikipedia article when I performed the search. (It's in the Wikipedia article now, much to my exasperation.)
An Internet search should be easy. It usually is! But because a swift, successful Internet search is part of my career, and my position as your humble author, I tend forget that things are often more complicated. Not this thing, oh my, no way.
OK! Let's do this!
The first, logical step, is to look at the soundtrack online. The above screenshot is of the soundtrack in iTunes (and playing the first song, "Pot Kettle Black" by Tilly and the Wall), but Amazon is OK too. Having located the soundtrack, listen to the provided samples of the songs to see if the song is on the soundtrack. Unfortunately, the song is not part of the soundtrack.
After this I checked Wikipedia to see if anything was available. I don't necessarily recommend Wikipedia as a fail-safe resource for Old People. (This is a big topic I hope to cover soon.)
The next step was searching for the song using Google. (Any search engine will do.) You can see I typed "whip it" end credits, using quotes around the film's title to narrow the search to whip it+end+credits instead of whip+it+end+credits.
The first result is a review from Variety. You can see text from the article beneath the link. The bold text indicates my search terms. The text indicates the end credits discussion is about the the visual imagery during the end credits.
The second result is an article from Movie Moron. The text here mentions a song in the end credits. The information provided is worth a click, and the article certain promises good news! The post is about the music used in the film! The post itself doesn't discuss the credits, but the comments do:
Poster DoubleD asks, "Who sings the song 28 at the end of the movie when they are showing the credits?" And two days later Karyn wrote, "Lorene Scafaria sings the song 28 whcih is featured in the end credits."
Scafaria is a screenwriter, actress, and singer who wrote an album during the Writer's Guild of America Strike.
That sounds promising! Let's look up Lorene Scafaria on iTunes (or similar service).
Wow. What a bummer. Let's try an Internet search:
Perhaps she does!
And here is Scafaria's MySpace page. Note that the song "28" is playing on the right, and her quote, on the left, to the right of her photograph notes the use of "28" in Whip It.
Oh my! That's the song you heard on Scafaria's MySpace page, which was, indeed, the song in question! Now the power to purchase the song is in your hands!