Just like you I was thinking about the possibility to lay my hands on additional 3D objects for my copy of Manga Studio EX. Though it comes with a whole bunch of 3D models that can keep you busy for a very long time you will come to the point where you want just that object that isn’t in the package.
On my prowl to find something new for you (and me) one of my mates sent me a link to a very interesting tutorial on Youtube. That shows you how to create 3D objects in Google Sketchup, convert them with Blender and then import them into Manga Studio. Both applications are free and easy to use.
All the credits for this tutorial are going to Christopher Chong over at Sonic Images who did a very good job demonstrating us how to do it: