Tate and Liza have on Ruby and Sapphire:
Solrock
Lunatone
On Emerald they have:
Claydol
Xatu
Lunatone
Solrock
It is on Mossdeep City. There are Twin Gym leaders
go into the gym
defeat the mossdeep city gym leaders and you will receive it after their defeat.
The sixth gym is Mossdeep City Gym - Gym Leaders Liza and Tate (Mind Badge). You have to surf right of Lilycove city.
Mossdeep cityit is in mossdeep
they have xatu lunatone and solrock. get a taillow and train it up into a swellow. it helps. or get a mawile
In Mossdeep City which incidentally is also home to the Twin Gym Leaders (Psychic Type)
To get to Mossdeep City, you have to defeat the other six Gym Leaders, and you need Surf. In Mossdeep, there is a Psychic-type Gym, and Team Magma will be in the Rocket place.
It could be anywhere between Mossdeep and Lilycove. The dark sea parts can be dived into but you need HM08, which is dive. Get it from Steven in Mossdeep (he's in the house at the top-left corner of Mossdeep). Just talk to him and he will give it to you. But you need to defeat the Mossdeep Gym to use dive. The Gym leaders use Psychic/Rock type Pokemon. You need a Dark or Water type Pokemon to have an advantage against them. P.S. Believe me, I had a Wailord in level 40 and I still had difficulty defeating the Gym leaders. Remember, the Gym leaders are twins so they use two Pokemon at the same time. SO put your strongest Pokemon at the top of your Pokemon list. Good Luck!
From Steven at his home in Mossdeep City but you can't use it until you've defeated Tate and Liza, the Pyshic Pokemon Gym Leaders :)
Then you will just fight with 1 Pokemon vs. Two foes Pokemon's Just like when you encounter a dual-fight with one Pokemon
Guess what you need at least 2 strong Pokemon because the gym leader is actually gym leaders meaning there is two that's because there twins so you have to have at least 2 strong Pokemon to verse them and win.