If you no longer have the standard assets .unitypackage on disk then you will need to download them again directly from the Asset Store. We no longer ship the standard assets in the 2018.2 installer, nor have they moved over to the new Package Manager system at this point in time.
Update 29th of July, 2020: Please note that since I originally wrote this guide, Unity Editor no longer has an option to download standard assets directly from the menu. Instead, go to Asset Store tab, then type Standard Assets into search and select the Standard Assets package (originally there was a menu in Unity Editor, in which you could automatically download standard assets. In newer versions of Unity, you have to use the store).
Unity 3D Comes with Standard Assets Pack that includes 2D, Characters, Effects, Vehicles, Utility, Prototyping, and some other packages. These assets let you use created prefabs, characters, effects, scripts, and other items that make it easy to make a game. There is no need to recreate something that has already been created by other users and made it available for free.
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