A reverse osmosis system lowers the total dissolved solids ("TDS") in water. If you install a deionization filter after your R.O. system, it will remove most of the remaining TDS from your water.
For a residential R.O. system, we have the Plymouth Products (formerly Ametek) PCF1-10MB cartridge. It is 2.5" in diameter x 10" in length. The price is $27.10 for the filter plus $14 for the housing with fittings, plus shipping. (No handling fee.)