Are you looking for a drinking water system or a whole house water filtration system? I will give you a couple options for both.
Reverse Osmosis systems reduce sodium from water. The RO membrane is the part of the system that reduces sodium. Here are a couple RO systems:
http://www.waterfilters.net/GE-Merlin-Tankless-Reverse-Osmosis-System_p_0-1467.html http://www.waterfilters.net/Good-Water-Warehouse/Reverse-Osmosis/systems/PuROTwist-4000.htm
For a whole house system which filters all the water coming into your house, you would use a whole house system with a deionization water filter.
Here is the system:
http://www.waterfilters.net/culligan/wholehouse/housings/20_BigBlue.htm
Here is the deionization filter:
http://www.waterfilters.net/Pentek/wholehouse/Specialty_Filters/BBF1-20MB.htm
Most people install two whole house systems together with a sediment filter first. The reason is that the sediment filter preserves the life of the more expensive 2nd filter which in this case is the BBF1-20MB deionization cartridge. If you do install 2 housings, the DGD-5005-20 is a very popular filter for the 1st whole house system:
http://www.waterfilters.net/culligan/wholehouse/replacement_filters/DGD-5005-20.htm
Thank you,
WaterFilters.NET