According to Sony, these demo kiosks will be gradually rolled out over the coming weeks, leading up to the PS4 launch. Sony Stores, GameStop, Target, Wal-Mart and Best Buy are most likely to receive kiosks first, though Sony claims that other retailers will also be participating.
This means nothing to you if there's not a participating store in your area, but helpfully Sony has also issued a list of which stores now feature PS4 kiosks. If you visit Experience PlayStation and enter your zip code, you'll be presented with a map of all stores carrying PS4 demo stations within your area. The system seems to be a bit glitchy at the moment - according to Experience PlayStation, the PS4 kiosk nearest my apartment is supposedly inside "The Wood Doctor," a hardware store specializing in kitchen remodeling - but Sony has assured us that this is a temporary problem.