There are few websites where you could buy them: www.airwaterice.com www.bulkreefsupply.com www.aquasafecanada.com <-this one was highly recommended, however, it is in Canada. If unit goes bad, all warranty repairs must be done i Canada you would probably be responsible for shipping there.
It is not the cheapest, but looks like it has all bels and wistles that you would ever need from RODI system. Also it comes as one unit with pressure gauges integrated, which makes it very solid one piece of equipment. <- but that's just MY PERSONAL impression.