https://www.thingiverse.com/ – Usually used for 3d printing stuff
https://www.models-resource.com/ – ripped assets from games!
http://www.turbosquid.com/ – For professionals (most cost $$$)
https://free3d.com/ – Large database, some paid lots of great free models
https://grabcad.com/ – more realism, its for mechanical engineers
https://3dwarehouse.sketchup.com/?hl=en – for sketchup (arch)
http://www.makehumancommunity.org/ – software for making 3d model
Email me at contact at charstiles dot com if you have suggestions to add 🙂 I bolded the recommended websites.