For those that water cool their PC's, if DI water in your area is costly or minimally available here's something I just remembered.
IF there is a local reef fish club in your area you may want to strike up a little deal with them. Most reefkeepers (especially of large tanks 75 gallons plus) normally have their own RO/DI water filtration system for their tap water. You could work out a deal with the club to do either some record keeping or a discounted build for them on a computer system, etc. (input imagination here) in essence getting free DI water for your PC cooling. I recently got rid of my 90 gallon and sold off my RO/DI system or I'd offer it locally here.
But I do know that most quality reefkeepers always use RO/DI so it may work out as another win-win.