Great source for RO/DI Gear

The Filter Guys - These guys really seem to know their stuff. Plus, their prices are very competitive with some of the big name brand RO/DI Units. When I get a RO/DI Unit, I plan on it being from them. You can get simpler models for significantly less than big brand models, and their high end models are far cheaper than the high end models from the big brands.

Good Websites for Aquarium Supplies

Big Al's Online - Good source of pet warehouse brand items for significantly less
Pet Solutions - Good source for pet warehouse brand items for less, with shipping typically isn't as good as Big Al's
Marine Depot - Good source for higher grade Marine Aquarium items for a decent price - carries better brands the other sites do not