Accseessing project assets from a custom editor

So i am working on a custom level editor for my game, that is a Unity Editor Script and not a runtime level editor.

Its basically just a row of buttons for easily creating the different level elements and a few useful functions like rotating and flipping objects.

My problem is how can i access the prefabs and button icons in my asset folder from a editor script? Preferably something where i can set up more buttons in the inspector, and not have to hard code each button.