crazysvn Flarum natively supports searching in tags. Either first navigate to a tag and type a query in the search bar, or select a tag in the sidebar while already on a search results page.
You can also use the tag: gambit for more advanced filtering, I think there's a page on this forum that explains the most common gambits.
However there's no way to search for tags outside of the tags selection modal, and I don't think there exists an extension for this.
