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.
www.freevectors.net
www.vectorart.org
http://vector4free.com/vectors
http://www.123freevectors.com/
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.