I'm not sure if your local stores will be getting any more. There were tons of converter boxes manufactured and sold last year as part of the big $40 government coupon program. This was done so that a lot of people could transition in time for the June 12 deadline when all major analog TV broadcasts were shut off.
Now that the transition is over and the $40 coupon program has ended, there's been very little interest in making any more converter boxes (some of the places I looked at list these as discontinued products). If you still want one, I imagine that you'll have to find a store that still has some excess inventory from last year that they are selling. There should be some around (perhaps even discounted) because there were many millions of these boxes made during the excitement leading up to the analog shutdown.
Some retailers still seem to support a few select varieties of converter boxes. For example, it looks like Radio Shack is still selling the Digital Stream DTX9950 (
link). Amazon still carries the Zinwell ZAT-970A (
link).
If you are interested in a box that can do the digital to analog conversion and also have recording capabilities (DVR), you might consider something like a DTVpal or a TiVo. These types of devices let you pause live TV, set up automatic recordings, stream video from select internet providers, and do much more than your basic converter box functions.