The only address I've found for the 5th Marine Division is out of date (individual deceased). This doesn't necessarily mean the association has disbanded. Except for a brief period during the Vietnam War, the division has been inactive since 1946 and those who served during World War II are rapidly disappearing. Harry B. "Harry the Horse" Liversedge won the Navy Cross (his second) commanding the regiment on Iwo Jima. He was a brigadier general, Director of the Marine Corps Reserve, when he died in 1951.
The address of the Division Scholarship Foundation is 260 South Norwinden Drive, Springfield, PA 19064-3517. It's unlikely any of the old-timers would be involved but the staff may be able to put you in touch with someone.