What kind of wikis? Here in Wikianswers, the search bar is on the right, below the toolbox, if you are logged in. If you're not, you need to use Special:Search. In the Monaco skin, used by most Wikia, the search bar is just below the logo on the left sidebar. In the Monobook skin, the default in Wikipedia, the search bar is often the one below the first set of links in its sidebar on the left, though its position can be changed by the wiki's admins.