I would first go to
www.3dcontentcentral.com as just stated for individual parts. You can select inventor format. If you want to see an example of a tetrix assembly then go to
http://blogs.solidworks.com/teacher and search for tetrix
You can download all the tetrix model files and the assemblies. I don't know how well SolidWorks assemblies open in Inventor but it is worth a try.
Good luck. Marie