The Thunder Charm can be synthesized for 5900 munny once Roxas is promoted to Novice rank.
Kingdom Hearts Re:coded
When Data-Sora equips the Thunder Charm, it activates the "No Jolt" ability, which prevents enemies from inflicting Jolt on you. It is obtained by giving the key item "Wrench" to Cid in Traverse Town.
The Wrench is obtained as a reward in the Traverse Town Alleyway system sector.