Does safari has an extension like this Chrome extension called Context Menu Search? It enables user to search highlighted text from the context menu.
I needed something similar too!
So look up: CustomSearch. It's highly adaptable by using the integrated CustomSearch Entry Editor.
"CustomSearch allows you to easily send the selected text to one of up to eight search engines — right from the toolbar or a contextual menu. CustomSearch includes search entries for Wikipedia, Google, Bing, and Amazon."