ALL-SILHOUETTES.com is the largest, online collection of free vector silhouettes. Most of presented silhouettes are free for personal and commercial purposes (see license information for each pack for more details).
Here is how to move an Illustrator shape into PowerPoint, not just as an image, but as an editable vector object.
In Illustrator export your shape in an EMF format
In PowerPoint, select “insert picture” (a bit counter intuitive)
Right-click the object and un-group it. Say “yes” to the question whether you want to convert it