Go behind the gym, surf across the water there and you'll find a cave.
in the dragons den after you win the last bag
go to the gym in blackthorn city, then surf behind it to the dragons den, it's in there
you can get a dratini from a guy in dragons den in blackthorn city or you can catch one in the water in dragons den
dragons den
gold and silver do not have a door to dragon shrine, there is no way to enter it