Good links for free 3D models

http://archive3d.net/
http://www.3dmodelfree.com/
http://tf3dm.com/
http://www.creativecrash.com/
https://clara.io
https://3dwarehouse.sketchup.com/
http://www.thepixellab.net/freebies
http://3dsky.org/
There are some free 3D models on http://www.cgtrader.com/, like this detailed microscope:

Available here: http://www.cgtrader.com/free-3d-models/science-medical/lab-equipment/microscope--6
And of course http://www.turbosquid.com/, if you look hard enough you might find some surprisingly good 3d models amongst the lower quality stuff.