The problem with free is, as the saying goes, you often get what you paid for :)
A good place to find 3D models (from the good to the unbelievably ugly) is the Google 3D Warehouse. It's a huge collection of free models that can be loaded with Google's Sketchup program plus be loaded in to Google Earth if geo-referenced.
From the site, here is a model of a boxing ring that can be downloaded in Collada format as well as for Google Sketchup. You ought to be able to find a way to convert Collada to OBJ.