I used to download basic shapes or make the in blender and bring them into UE, while all basic 3D shapes are included in the Engine folder(such as arrows, hexagon, hollow tringle etc)
And many material functions and basic materials as well.
Nowadays I sometimes just go through the folders in the Engine Folder, or sometimes plugins folder, and just look through them to find something interesting.
I sometimes click on Engine folder and filter it as static mech, or texture, or materials or skeletal mesh etc and find great new things. I found the festival stage 3D assets the same way.