After having finished watching season one of Boston Legal last week (William Shatner is just great), I went shopping today to get season two. But no luck. Neither did I find Ally McBeal season one, or Grey's Anatomy season one, although season two of that was stacked high in the store. I ended up with M*A*S*H season one, and The Shield season four.
Two or three years ago there were very few TV series on DVD available, and a DVD store basically had them all. Now more and more are coming out, and shelf space is limited, so not everything is stocked any more. It's getting to be just like books, where a bookstore only has the bestsellers available, you'll need to order anything else. The large number of not-bestsellers is called the "long tail", and is often quoted as one of the success reasons for Amazon.com. Guess that is where I'll be heading to buy my DVDs.