You can catch Dratini/Dragonair outside dragon spiral tower by using the super rod in the water. If there is a bubbling spot you can even catch a Dragonite if you use the super-rod on it(Warning said Dragonite can be from level 50 to level 70 it will not be easy to catch).
Old-rod: Not available in B/W
Good-rod: Not available in B/W
Super-rod: Nuvema Town given to you by Looker in your house after you beat the main story line.
Sources
Dratini
Rods
