Hannya, foo dog, skull, octopus, etc are not very common, but I do say things are common for a reason (they look good).
When researching what to get for a Japenese tattoo I suggest looking at old Japanese artwork (woodblock prints), as this is the same source for tattoos.
Amazon.com: Japanese Woodblock Prints: Artists, Publishers and Masterworks: 1680 - 1900 (9784805310557): Andreas Marks, Stephen Addiss: Books
Amazon.com: Masterpieces of Japanese Prints: Ukiyo-e from the Victoria and Albert Museum (9784770023872): Rupert Faulkner: Books
Amazon.com: Japanese Prints: The Art Institute of Chicago (Tiny Folio) (0735738061374): James T. Ulak: Books