Do you mean an on-line merchant?
If you're plannning a trip to Tokyo, you can clean up on discontinued titles strolling up Hakusan Dori between Yasukuni Dori and Kanda-Gawa. A shop which has many former rental copies of AVs is L I N K at Misakicho 2-7-11, Chiyoda-ku. Subway: Jimbocho.