Found this list on a facebook page. Removed the deadlinks and
Lithos
3dp.rocks
itslitho.com
lithophanemaker.com
CAD software
cookiecad.com
Objects
3dcadbrowser.com
3dcontentcentral.com/
3dexport.com
3dkitbash.com
nih.gov – 3d
cgtrader.com
cults3d.com
crealitycloud.com
wixsite.com – dtrstudio3d
fab365.net
free3D.com/
free3D.io/
zortrax.com – library
nasa.gov – nasa3d.arc
nikkoindustries.com
pinshape.com
Printables
repables.com
sketchfab.com
spriters-resource.com – models
redpah.com
Threeding.com/
thingiverse.com/
Tinkercad.com
Treatstock.com
turbosquid.com
yeggi.com
YouMagine.com
Indexer
stlfinder.com
three-drop.com