I'm not surprised by the "low demand"/can't-find-it-anywhere dilemma. Foster's, despite reportedly being one of the highest-rated originals on CN, is unfortunately not popular or well-known in the mainstream on the same level as SpongeBob or Fairly OddParents or Avatar(I've read repeatedly that Nickelodeon almost always crushes CN in the ratings). I'm crossing my fingers that the local Borders will have it, since they have a far more expansive and varied catalog of DVDs.
But anyway... I checked the DVD page, and I must say it looks GREAT.

I really want to listen to the Store Wars commentary track. This DVD and the upcoming Frankie comic-book will definitely keep me happy until whenever Season 5 is ready to debut.