Thingiverse is a 3D print and design community where talented modellers post their parts, toys, gadgets, tools, art, and all sorts of things ready for 3D printing.
Before you start modelling or designing, check Thingiverse to see if somebody has already shared something similar. All of the models are free to download and are shared under Creative Commons licenses. Make sure to look at the specific license for each model to determine what you can do with it; some let you change and build upon them if you give credit to the original author.
Alternately, consider uploading your finished design or work in progress to Thingiverse. The open source 3D printing community thrives on freely sharing knowledge and information. If you think your design could be used or admired by other people, sign up and share your model with the community.