http://www.deviantart.com
http://www.skinbase.org
http://www.skinnalicious.com
http://www.winmodify.com
http://www.artnskins.com
http://www.customize.org
http://www.skinning.net
http://www.skinartistry.com
Those should get you started ...each of those sites will most likely have links to other sites where you can continue your search.
Happy hunting
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