Whois wildcard query
The best search engine I know of for this kind of search is the one provided by N@me Droppers. Type your trademark in the search bar and use the pull down to select "Only Registered Domains".
To test it, I put in "superuser" (in .com only) and it found superuser.com, androidsuperuser.com, asciisuperuser.com, asksuperuser,com, beasuperuser.com, superuserfilter.com, and so on.