have you tried disney's website? try the walt disney world website too as there are stationery shops in the parks that sell paper with the characters on it.
a friend of mine went for Kermit and Miss Piggy (don't ask) they found that the best way was to go to a company who makes invites with a picture of what you would like so that they can download it onto the invite. I would go to a disney website, find the pic you want, save it and send it to the stationer... They should be able to do it for you!