How do you get the sappire for trading pokemon from firred to the old versions. What island do you get it?

Answer 1

after you first fight a specific rocket member, he'll give you 1 of two passwords that you need to get into the rocket warehouse.

in dotted hole, if you chose the correct path (instead of only the wrong paths for prizes) you'll find the sapphire. but a scientist will steal it, and drop password number two

go to five island. you'll find the rocket warehouse. it can be tricky if you don't pay attention, but at the end there is a fight with a scientist, and you'll win the sapphire