Find all four seasons in iTunes here. Or if you're more of a DVD person you can find seasons 1-3 on amazon. (Season 4 should be out on DVD later this year.)

In other releases:
Veronica Mars Season 1 is in iTunes in Standard Definition and High Definition. (Probably one of my all-time favorite seasons of any show.)
There's also a collection of 10 Degrassi episodes you can get as "Degrassi Love 101" in iTunes.
They've also got the cover of the complete Daria series (coming out May 11) up on Amazon. (49 5-star reviews and the DVD aren't even out yet, You rock on Daria!)
Here's a long list of CW shows that have been renewed.
And here's an annoyingly cryptic Glee promo.