Try looking at your local comic book stores. Sometimes they have special editions or box sets available. Also, check the official publisher's website for direct purchase options.
Well, you could try looking at specialized manga retailers' websites. Some well-known ones include Right Stuf Anime and Book Depository. They often have a wide range of manga box sets available. Also, check out conventions or comic book fairs; sometimes vendors there sell such items.
Well, major bookstore chains like Barnes & Noble might carry it. Or you could try searching on the official Naruto manga website to see if they have any purchase links or recommended retailers for the box set.