I just bought this one from the same seller after looking all around.
150GPD 0ppm Reverse Osmosis+(2)DI Aquarium Water Filter - eBay (item 110477085307 end time Jan-10-10 19:40:01 PST)
...many others on bulkreefsupply.com, etc, but IMO
they're all the same. BRS looks like they use better quality filters and fittings, but you can't beat the price. I went with the 150 GPD because I maintain 2 tanks in addition to my own, and it's going to supplement my household system.
Many others on this site have used the same units, I haven't heard anything bad. I would have liked to see the 0.6 micron filter stage that the BRS units use, but I can buy that when I change out the filters.