You may be able to source materials associated with no fee from the following

Wikimedia Commons http://commons.wikimedia.org/wiki/Main_Page . Only material made available under a Creative Commons licence or released as “public domain” may be used.

Www.openclipart.org
Www.wpclipart.com
Www.pdclipart.org
Www.clker.com

Illustrations from these sites may be used for free without attribution.