When needing a fletching knife in old school RuneScape, players have the following options.
Knives spawn in Lumbridge. There is one in the southwest corner of the kitchen, one downstairs, and one behind the ax shop. A knife spawns in a house behind the Catherby bank and in a house southwest of the Seer's Village bank. A knife and a spade spawn upstairs in the Varrock general store.
Knives can be purchased in the shantay pass shop, from Ali Morresette in Al Kharid, and in the Catherby store. Other stores that sell knife are the Ardougne spice stall and Heckel Funch's shop in the Grand tree. A knife spawns in the Musa Point general store.
RuneScape players should stockpile several knives, as this item is often sought out by players in a bank who can't be bothered to find one and are willing to pay a high price to have one right now.
Old School RuneScape starting out guides main link list hub.