I have zero experience with any type of 3D modeling software. So, you can understand my apprehension when I wanted to make my own custom 3D printed object. Luckily there was a very detailed Instructable already written on how to make your own action figure that outlined the process, showing that mashing up existing 3D models is easy. Making my own custom 3D model was so simple I was able to make my model in about an hour on the computer.
This project uses the free modelling tools Netfabb, and MeshMixer. I printed my model through another service, but there's currently an offer through Instructables to print a model for free! You could make your own for almost nothing! How great is that?!
Step 1: Find models and convert with Netfabb
123D Gallery and TurboSquid.
If you've downloaded .STL files you'll need to convert them to .OBJ files before you can edit them (if you have .OBJ files then skip to the next step). After you've found the models you want open each model separately in Netfabb and export as an OBJ file: Part > Export Part > As Wavefront OBJ. Next, we'll combine the models in MeshMixer.