The ark file is an encrypted package that is inaccessible to view the contents for licensing reasons and also can only be used in the editor. The .pak can be used in end game and editor
.ark & .pak files are automatically loaded into the engine when they are placed in the "Asset Library\ARK Files" folder. We tend to store a preset of the meshes that need to be used as that will load the asset into the engine rather than not being able to open the content to drag it in the scene.
Anyone can create their own archives from the tools menu in the main menu which is apart of the main editor.
As for assets, we have been around many asset stores across the web and purchased a few to use in our games/tech demos/examples.
Such sites like: Arteria3D, Dexsoft, 3dFoin, turbosquid as Shando mentioned.
Even then, there are still more and if licenses permit it, then stores from other game engines can be used if the assets are in a .FBX format.
Hope this helps