How do I search for Safari Extensions?
There are many extensions not listed on Apple's page. However, if they're not on the main page, they're not on Apple's site at all. Your cmd-f solutions is the best option if you want to search just the extensions that Apple has listed.

If you want to broaden your search to more of the extensions available, turn to third-party sites: http://safariextensions.org/, http://safariextensions.tumblr.com/, and http://www.pimpmysafari.com/category/extensions are all good, more-comprehensive lists.