Therian Formes in ORAS?

Question

How do you get the therian formes of the weather Pokemon in ORAS?
Don't I need a Reveal Glass?
If so, where is it/how do I obtain it?

Comments

There's a mirror shop in Mauville City. Show then one of the trio and they'll give you the Reveal Glass

Answer 1

In Mauville City's main area, you will find a special mirror shop on
the left hand side. If you show them a Tornadus, Thundurus or
Landorus, they will give you the Reveal Glass item which allows for
you to freely change form between each of the Pokémon

Source