Mike's Tanks identifies some Dragon Tanks as "retired", scroll down the page till you see the chart:
http://www.mikes-tanks.com/DragonArmor.htm
Of course the word "retired" is not the best word to use, since it could apply to any tank that is no longer being produced at the factory. Or perhaps "retired" means not available to retail stores from the wholesalers.
99% of the Dragon tanks are available, if you're willing to spend a lot of money! I've seen some of the rare tanks sell for close to $200 on ebay.