There are websites that offer 3D models in both dxf and 3DS (and other) formats.  One such is http://www.3Dcafe.com ;

You get varying degrees of "quality" - some of the meshes are detailed, but others aren't as detailed, and lack landing gear etc.  Many are made as artistic models, and not flightsim specific.

Freeflight Design Shop (http://www.freeflightdesign.com) has a number of gmax source files, and all non-gmax source files (dating back to 1998) can be converted to dxf upon reasonable request to the FFDS website manager.

One of the neat things about these "older" AFX models is that they provide a modeller as a useful 3D "backdrop" against which to rebuild a model.