In New York (and probabaly other metro areas with high concentrations of Japanese people) there are Japanese used book stores. The one we go to is called Book-Off. A major portion of their trade is comic books (or you can call them manga if it makes you feel better).
They might have back issues of Shonen Jump. Check phone directories. There's one in Manhattan, I think on 40th or 41st near 5th Avenue. There's another in either Hartsdale or Ardsley. (I'm assuming they'd be willing to ship)