DecadeCounter.com has a new kit and clock for sale on their website - it is a single digit numitron clock. I really like the small design of the kit and you can't go wrong with the price. Check it out. Lastly, check out Accutron's post on the forum if you have any questions.
The YS-950 is an inexpensive single-digit clock equipped with a 'Numitron'-style seven-segment filament display tube and a 16F628 PIC microcontroller. The YS-950 displays time by cycling through each digit incrementally, starting with the tens of hours position and ending with the minutes position. A row of four LEDs along the side of the device indicate which digit is currently displayed. The display will remain idle for a user-defined amount of time between each digit display cycle.