RuneScape Classic Wiki
RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Mithril Short Sword
The Old School RuneScape Wiki also has an article on Mithril Short Sword.

A Mithril Short Sword is a short sword which requires at least level 20 attack for players to wield. It can be purchased from Varrock Swords, obtained from monster drops, or made from one mithril bar by players with at least level 54 smithing, yielding 50 smithing experience.

Dropped by

They are dropped by:

Monster Combat level Quantity Rarity
Moss Giant 62 1 4Rare