Thank you Mike Edelen for sharing this info (3D printing FB page):
JUST A FEW SITES FOR STL FILE downloads:
https://printables.com
https://thingiverse.com
https://Cults3d.com
https://sketchfab.com
https://pinshape.com/
http://www.3dshook.com/
https://grabcad.com/
https://www.cgtrader.com/
http://www.turbosquid.com/
https://3dexport.com/
http://www.yeggi.com/
https://www.remix3d.com/
https://fab365.net/
www.tinkercad.com/things
www.cgtrader.com
www.forge.zheng3.com
www.stlfinder.com
www.trinpy.com
www.digible.net
www.shapeways.com
www.yobi3d.com
www.youmagine.com
www.3dwarehouse.sketchup.com
www.3dcadbrowser.com
www.123dapp.com/gallery/content/all
www.libre3d.com
www.3dshook.com
www.3dexport.com/free-3d-models
www.tf3dm.com
www.treatstock.com/3d-printable-models
www.open3dmodel.com
www.archive3d.net
www.3dcontentcentral.com
www.shapetizer.com
Gumroad.com
https://www.stlfinder.com/
https://3d-gallery.xyzprinting.com/en-US/promotion
https://3dprint.nih.gov/
https://3dexport.com/
https://repables.com/
https://www.instructables.com/
https://libre3d.com/
https://3dkitbash.com/
http://www.rinkak-services.com/en/
https://library.zortrax.com/
https://fab365.net/
https://polar3d.com/
https://free3d.com/
https://www.threeding.com/mobile
https://nasa3d.arc.nasa.gov/models
http://zheng3.com/forge/index.php?id=-4
https://www.redpah.com/
https://digilab.dremel.com/resources/lesson-plans
https://www.3dagogo.com/
https://all3dp.com/.../free-stl-files-3d-printer-models.../
https://www.malix3design.com/?m=1
https://dtrstudio3d.wixsite.com/website
https://www.zsculptors.com
thangs.com
Cookie Caster - http://www.cookiecaster.com/
Lithophane - http://3dp.rocks/lithophane/
itslitho.com
Instructables - http://instructables.com/
NASA 3D Resources - https://nasa3d.arc.nasa.gov/
3D Warehouse - https://3dwarehouse.sketchup.com/
Smithsonian - https://3d.si.edu/
ShareCG - http://www.sharecg.com/
All3DP - https://www.all3dp.com/
3DExport - https://de.3dexport.com/
3DContentCentral - https://www.3dcontentcentral.com/
3DShook - http://www.3dshook.com/
3DExport - https://de.3dexport.com/
3DContentCentral - https://www.3dcontentcentral.com/
Print a Brick (Legos) - https://printabrick.org
Repables - https://repables.com/
SketchFab - https://sketchfab.com/
Tinkercad - https://www.tinkercad.com/
Evermotion - https://evermotion.org/downloads/
Blendswap - https://www.blendswap.com/
3DModelfree - http://www.3dmodelfree.com/
Yobi3D - https://www.yobi3d.com/
ThreeDscans - http://threedscans.com/
BlankRepository - https://www.blankrepository.com/
3ddigitaldoubles - http://3ddigitaldoubles.com/
Pixellabs - https://www.thepixellab.net/freebies/
Do3d.com
3Dmag.org
https://www.autodesk.com/products/fusion-360/overview