Most water delivery services have distilled water which would be very similar to RO/DI.
Personally I would look at buying a reef quality RO/DI system as with a tank that size you will go through a lot of water and the system will quickly pay for itself. RO/DI does not come any better than SpectraPure Water Purification Products who have been doing just that for over 25 years. They have a 150 GPD Reef Quality RO/DI system with a handheld TDS meter on sale right now for $150 that is hard to beat or my personal favotrite which I own myself, the MaxCap dual DI for only $269, this is the Cadillac of all RO/DI systems and what other vendors strive to copy.
http://www.spectrapure.com/email/cus...reciation.html