In the tunnels of Varrock their is a knife you can fine after a little searching its near the giant spiders
ADDED- you can buy one at the ge most general stores behind lumby castle and many other places. Knifes are very common.....You can also get a sacred clay knife from stealing creation for double exp for a limited amount of fletching. Added- In the Lumbridge kitchen, go down the trap door. There is one in the cellar.