Duck & Fox:
https://github.com/KhronosGroup/glTF-Sample-Models
Cesium:
https://github.com/CesiumGS/cesium/tree/master/Apps/SampleData/models
https://github.com/CesiumGS/cesium/tree/master/Specs/Data/Models
FlightRadar:
https://github.com/Flightradar24/fr24-3d-models
Kenney:
https://kenney.nl/assets?q=3d
Smithsonian:
https://3d.si.edu/cc0?edan_local=&edan_q=&
AssetGen:
https://github.com/KhronosGroup/glTF-Asset-Generator