Some sources of RO or RO/DI water:
-- LFS often have RO/DI water by the gallon
-- Those large water dispensers usually found outside/inside of grocery stores, Wa-Marts, etc. I believe most even post when the filters were last changed.
-- Your own in-home RO/DI unit. A good unit will take even very high TDS counts down to zero. Cost can range from around $100 to $200 and up. The advantage is that you will always have water, no running out (Unless you empty your resevoir container), no having to gather up a bunch of buckets and head down to the nearest outside source, and so on. These units can easily pay for themselves (Especially if you factor in things like gas, and time/effort/hassle).