By making them. You can create 3D models with blender,maya or 3dsmax. For 2D I would suggest gimp or photoshop.
Gimp&Blender are both free.
how can I get more assets?
Here you can download more assets:
- ://www.blendswap/
- :///
- ://tf3dm/
- the demos from epic games
- ://www.cgtextures/
- ://texturelib/#!/category/?path=/Textures/rock/stones
- or as DoctorPC said, create them with a 3d program
