Whether just starting out with 3D modeling or looking for something specific for your AR project, these are a few good sources to acquire 3D asset files from:
Turbosquid - https://www.turbosquid.com
FreeCAD - https://www.freecadweb.org
CGTrader - https://www.cgtrader.com/
Grab CAD - https://grabcad.com/library
My Solidworks - https://my.solidworks.com/cadmodels/explore
3D Content Central (Dassault) - https://www.3dcontentcentral.com/
Blender 3D Models - https://www.blender-models.com
https://www.blender-models.com/model-downloads/vehicles/id/spitfire-mk-xiv/