You need to buy it from the game corner

Oh dear, any Dragon Type? Well... The only one that stays pure Dragon Type even in it's final evolution is Axew, which you get via trade. Dratini is a rare find in the Safari Zone, and can be bought from the Game Corner, but upon its final evolution it gets a secondary Flying Type, which makes it take normal damage from the Electric Type, double damage from Rock, and quadruple damage from Ice. Ouch.

Those are the only two I know where to find off the top of my head, unfortunately... I would recommend taking a look at the PokeUE wikia to find the rest.

Dratini's a 1% chance to appear with the Super Rod in Safari Zone in Tiers 1 and 2, Dragonair appears in Tiers 3 with the same requirements