Please specify what the plugin you recommend does. Plugins of interest include those that provide access to http://search.creativecommons.org as well as those that work directly with CC-filtered search at specific providers, eg Flickr and Google.
You can also explore other available OpenSearch plugins related to "creativecommons".
There are multiple ways to add custom search engines to Safari. The URL for a CC Search query is http://search.creativecommons.org/?q=<search query here>: