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

A Large Mithril Helmet is a helmet which can be worn by players with at least level 20 defense. It requires level 57 smithing to make from 2 mithril bars, granting 100 smithing experience. It can also be bought from Peksa's Helmet Shop in Barbarian Village for 1430 coins.