http://www.watergeneral.com/product01.aspx

They sell the RO model RD-102/106 on ebay. I have them. They have very good rating for water quality on Reefcentral and cheap also.
I tested my water from the RO unit once every month, it's at 0 ppm on an expensive TDS meter.

I hightly recommend this. I have the RD-106, (have drinking water option that I don't use, by Ozaka), but
for aquarium, an RD-102 model is all you need.