You'll have some luck looking for them at the Google SketchUp repositories (user-submitted content). The repository is also known as Sketchup 3D Warehouse.
For instance: http://sketchup.google.com/3dwarehouse/search?q=arduino&styp=m&scoring=t&btnG=Search
Once you have Sketchup installed, you can download and use these models and possibly export them to a format known by another 3D package, such as 3D Studio and so on.