RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Large Iron Helmet
The Old School RuneScape Wiki also has an article on Large Iron Helmet.

The Large Iron Helmet is the second weakest large helmet in RuneScape Classic, preceded only by the Large Bronze Helmet. There is no Defense requirement to wear this item. A player can obtain a large iron helmet from Peksa's Helmet Shop in Barbarian Village or by making one with at least 22 Smithing. In order to make one, players will need two iron bars and a hammer. The player will receive 50 Smithing experience from making a large iron helmet.

Item spawns

Location Number of spawns Number in stack
Ground floor of the Varrock museum 1 1*

*This particular stack may only be obtained by casting Telekinetic grab on it with Level 33 Magic

Dropped by

Monster Combat level Quantity Rarity
Black knight 46 1 2Common
Giant 37 1 3Uncommon
Jailer 51 1 7Unknown