Free vector art sites can provide a valuable source of clipart that can be cut with a vinyl cutter and integrated into signs and graphics designs. Silhouettes are fairly easy to find in vector format and if necessary are easy to convert from a jpg format to vector with most sign design programs. Here are some links to free silhouette vector art and clipart for signmakers.
Some of the vector graphics on these sites may be difficult to open with your sign program or via adobe illustrator. A solution is to bring the file into adobe photoshop, convert it to a black and white image, bring it into your sign program, and do an autotrace. Then you just have to clean it up a little.