RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Mithril Plated skirt
The Old School RuneScape Wiki also has an article on Mithril Plated skirt.

The Mithril Plated skirt is a plated skirt that can be made using 3 mithril bars at level 66 Smithing, yielding 150 experience. It requireslevel 20 Defense to equip.

It can be exchanged for Mithril Plate Mail Legs from Thrander in southeast Varrock, and can be bought from Ranael's Plateskirt Shop located in southeastern Al Kharid.