FYI, any real OD Elcan Sleeves that become available, will most likely be acquired via unlawful means, as the "OD Green" rubber cover is considered a controlled item.... as for a black replacement cover, you can either try contacting Armament Technology, they are ELCANs reseller, or try contacting ELCAN directly.
http://www.armament.com/
http://www.elcan.com/