You can find free and paid 3D models in .obj and other formats at these websites:
CGTRADER - diverse types of 3D models
GRABCAD - diverse types of 3D models
HERMAN MILLER - limited to model types usually not directly usable in Blender, however you can convert them from .skp and .dwg . They used to have 3DS formatted models there, which could be brought into older versions of Blender directly, but I don't see them there now, and with Blender no longer able to do .3ds imports, I guess they won't be missed much?
KOHLER - Free 3D models of thousands of plumbing fixtures and bath related products in .obj and other formats.
TURBOSQUID - diverse 3D models
SKETCHFAB - diverse types of 3D models