There are many places to find 3D models on the web. Some are digital libraries full of user generated or specially curated models. Others are web apps that allow you to create 3D models right in your browser window. Below is a list of some of the 3D model resources to get you started.
Keep in mind there is no guarantee the models from any of these websites will print successfully, especially the models submitted by users. You will have to rely on your knowledge of 3D printing to decide which models will and will not print.