Manganese nodules are rock concretions on the sea bottom formed of concentric layers of iron and manganese hydroxides around a core. It is not easy to get Manganese nodules as they are protected by the International Seabed Authority, which has the responsibility for controlling all deep-sea mining in international areas, and protect the marine environment from any adverse effects. Nevertheless, you can find some Manganese nodules on e-Bay. Try this site if you are really interested in buying some. You can also try any Gem Fair where sometimes it is possible to find exotic stuff like this.