Tortoise shells are dropped by Tortoises and Warped tortoises. They can also be received by completing daily challenges. Both monsters can drop up to three shells at a time per kill. They can be sold to Barlak in Dorgesh-Kaan for 250 coins but sell for much higher on the Grand Exchange. They are also used to make War tortoise pouches for Summoning thus making them highly valuable—war tortoise familiars are common familiars for high-level players to use, due to their ability to store 18 items and the fact that summoning them only requires 67 Summoning.
Collecting shells can be a reasonable way of making money, when combined with the other valuable drops from Tortoises or Warped tortoises.