There are DOZENS of trailer houses that work on everything from small indie films to the biggest Summer blockbusters.
You can find the names of just about all of them in the Golden Trailer Awards' directory page! Most of them have Twitter accounts with relatively few followers, so give them some love.
Also, follow @Trailerbeat, and @TV Beat, whenever a new trailer or TV spot comes out, they do their best to find which house cut it.
Unfortunately there's no Trailerbeat equivalent for game trailers, but you can find out names of various agencies via the Game Marketing Awards nominees.