Drop in to a local saltwater aquarium store. They will have a huge tank of RODI they sell people. It was pretty cheap at the time, 5 gallons was something like $10. It was bring your own container, but those 5 gallon water bottles are pretty cheap and reusable.
The filter to make it is also not too bad. It's just a normal reverse osmosis setup with an extra post-RO cartridge for the DI stage. If you already have a filter, you can get a standalone canister and add the DI on a valve to re-use the existing setup. I remember reading at the time that the DI stage was not guaranteed food-safe, so they didn't recommend putting it on a drinking water setup. Might just be laywer stuff, but just in case. Ultra pure water isn't great for you to drink anyway. Drinking systems usually have a mineral cartridge for that and taste. So you can pay to strip it all out, and to put some back in!
The closest salt water aquarium shop is 44 miles so 88 miles round trip.
The Distilled water worked beautifully and it was 18 miles away so 36 miles round trip and cost $1.37 a gallon. I'll most likely use 5 or 6 gallons a year.
I know you guys want me to be successful. The laws of diminishing returns say a super nice burn with very little expensive is good enough.