Sorry I didn't see your questions until CincyCichlids bumped the post. Hopefully it'll help you or someone else out.
If you're in Springfield, there's a place called Sparkling Carbonics off Glendale-Milford Road. They have the best prices as far as I know on CO2 cylinders. Depending on your tank size, number of tanks, bubble count, and how much room you have, you can decide on what CO2 cylinder to get. I use a 10# cylinder and the thing lasts forever powering my 10, 20, and 29. I believe a cylinder from there was $50, and $8 or something for a refill. I only saw steel tanks there, but they might give you a shiny aluminum one if you ask nicely.
Also up north in Hamilton and also down in Cincinnati, there's a chain called Valley National Gas. They're more expensive, but I've had good luck with them too. I got a nice shiny aluminum 5# I later traded in for my 10#.
As far as RO/DI in freshwater...I think your answer is maybe. I *think* some discus breeders might use pure RODI for their tanks, or atleast pure RO. However, it probably depends on what you're keeping. What I do in my shrimp tanks is start out with pure RODI water, and add GH Booster (you can buy it online from aquariumfertilizer.com) or Seachem Equalibrium to get my GH to the desired target, then add Baking Soda to raise my KH where I want it. This way you can control your own watery destiny.