RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Lobster
The Old School RuneScape Wiki also has an article on Lobster.

A Lobster is a fish that players can use as food. When eaten, the player is healed 12 Hits. A lobster can be obtained by Cooking a raw lobster at level 40 Cooking, granting 120 cooking experience. Cooking it unsuccessfully returns Burnt Lobster. Players stop burning lobsters at level 74 cooking, or 64 cooking with Gauntlets of cooking. Like many fish, lobsters can be exchanged for lobster certificates by talking to Niles at the Draynor Village market.

Dropped by

Monster Combat level Quantity Rarity
Fire Giant 109 1–2 2Common
Black Demon 156, 175 1–2 2Common
Pirate 27 1 4Rare

Sold in

Shop Location Stock
The Shrimp and Parrot Brimhaven 3