Tortoise shells are dropped by Tortoises. They can drop up to three shells at a time per kill. They can be sold to Barlak in Dorgesh-Kaan for 250 coins.
Sometimes tortoises can also drop Perfect shells, a rarer counterpart to the Tortoise shells. Like the regular version of the shells, a player may receive up to the 3 of these shells. Each perfect shell can be exchanged for 600 coins and 500 Crafting experience from Barlak in Dorgesh-Kaan.