I get most of this sort of stuff from the Google 3D Warehouse. I believe you can import these with the appropriate add-on (Windows only unless that's changed recently). I use SketchUp Pro to export as 3DS which works well but only if you have access to the Pro version (you can download a demo copy that will run for 8 hours if you just need the one off).
There are also quite a few sites where you can look for ArchiCAD specific parts. There are lists of link sites but I can't seem to put my finger on one right now.
There are also 3D models available for a fee from places such as Turbosuid and FormFonts (which has a couple of fire engines). Typically these are offered in 3DS which you can import if you have the add-on loaded.
