As noted above, some artists just aren't on iTunes for one reason or another. They just got the rights for Jethro Tull's music a few months ago, there's no AC/DC at all, and no Seger to speak of.

That said... Russ found me a bunch of Seger on the torrents, including the original single release of "Ramblin' Gamblin' Man," which is much rawer and cooler than the live version that he released later. Drop me a line if there's anything special you'd like.