One of the best sites is http://vectorart.com
as they're designed to work with CNC machines and are clean vectors.
Most vectors are usable though, some with more work than others, and tracing (either auto or manual) works well with line drawings, silhouettes etc.
Google Image search is your friend here (https://images.google.com/
) type in a search term and then you can refine the type of image (line drawing, vector etc) using the boxes at the top of the page.