There are many different programs you can use to make your own 3D model or to alter a pre-existing model's file. Just make sure that the software you chose can exports files in the STL format or that you install an extension that will allow to create files in this format.
Below is a list of free design software:
There are many free and for-purchase software programs for you to use. A more complete list of free softwares can be found at 3ders.org.
Don't want to make your own model? There are many websites that host files of 3D objects that you can print. Here are some of these websites:
There are many places around the web where you can download 3D model files. A list of more webistes, including the ones above, can be found at 3Dprintingforbeginners.com.