Unless you're a capable 3D designer, you'll have to find pre-made 3D models to send to your 3D printer. This can be problematic for a couple of reasons: searching for non-textual items is always difficult; there are an increasing number of 3D model repositories to search.
The latter is partially solved by the strangely named Yeggi service, which provides a meta-search over several 3D model repositories. It's not clear how many repos are searched by Yeggi, but they definitely cover Thingiverse and Ponoko, two of the largest sources.
The problem facing searchers is that many 3D model repositories on the Internet were originally designed to hold visual 3D models that are not suitable for 3D printing. Yeggi presents printable models only.