ETA: Corrections made thanks to darkoshi for being awake when I wasn't
(for darkoshi and everyone else who'd rather not log in to view profiles or search results - myself included)
Pinsta.me doesn't have all profiles cached. Searching is more likely to return an overview of all pages containing your search query than to point you to an official profile (if an official profile is what you're after).
Looktagram, despite advertising itself as "@user and #tag search", has no search functionality. You'll have to use it the old-fashioned way by appending your search terms to the URL. Example: http://looktagram.com/mariah