I have never played this game so I'm not certain this will be helpful but most video game problems are easily solved by visiting http://www.gamefaqs.com
There is suggests that a gabite scale can be acquired using something called a wonder mail code which is listed below.

NJJQ 2XPM 8KM+
K6JS 4#J9 0C+K

In case you don't know what wonder mail is here is a description copied from gamefaq.

WHAT IS WONDER MAIL?
What is Wonder Mail you ask? Wonder Mail is a way for players to give missions or rescues to other players via password. Every rescue has a Wonder Mail password written at the bottom of its description. You simply imput the code of the mission before you start the game, at the bottom of the menu and you will have the mission added to your Job List. The code works on any and every Pokemon Mystery Dungeon game. You can't, though, use the same code twice on the same game.
Source(s):
http://www.gamefaqs.com/portable/ds/file/938931/53054
http://www.supercheats.com/gameboyadvance/walkthroughs/pokemonmysteriousdun...

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The Gabite Scale can be found in the Maze Cave.

"Maze Cave

* Boss: Gabite
* Floors: 10
* Notes: Cannot recruit Gabite
* Objective: Save Shinx' sister with a Gabite's Scale.

This dungeon can be entered before going to Mystic Forest. The Pokémon are not too hard; they're about as strong as Pokémon on Mt. Spike. Just keep going until you reach Gabite.

* Gabite
* HP: 300+
* Ground/ Dragon
* Average Damage: 10+

By comparison, Gabite is stronger than Drowzee, but by this time, you should be strong enough to defeat him. Just keep pounding him with good attacks, and he should go down like a lead balloon."
Source(s):
http://strategywiki.org/wiki/Pok%C3%A9mon_Mystery_Dungeon:_Explorers_of_Tim...

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