marine salt mix! the base of SW aquariums! sheesh.... you buy can buy it at any LFS.. just ask for marine salt mix... read your article for more info on salinty.
You use normal marine salt (like you will be using for your marine tank you created a thread about) and mix it to a lower salinity than you would for a normal marine tank. You measure the salinity with a hydrometer (wich you will also be using for your marine tank).
You cannot make and/or simulate brackish or marine water with just sodium. There is a host of chemicalls and/or minerals (link in marine and brackish water. Sodium is only one chemical/mineral - you need A LOT more than plain sodium if you want to make brackish or salt water wich is capable of suporting BW & SW fish and inverts.