If you want to make your own (and live in the USA) take a look at the stills made by this company: <
Aqua Clean>
Yes, they are expensive, but I am amazed that this company still makes such a high quality product in the USA. A friend gave me a small table top model that was several decades old (now discontinued). Not only does it still work, but the company still stocks replaceable parts for it!
Where I live I can usually find di (deionized)/distilled water for about a dollar a gallon (last time I checked) - and I use only a few gallons per year. So I am not going to live long enough to break even on the purchase of a $600-700 still. But if you need several gallons per week, it might make more sense(?)
About all I use di water for is to make up ascorbic acid type film developers and for dilutions of those developers. And also as a final rinse for film. (We have hard water here where I live - plenty of calcium to form spots, and maybe enough iron to worry about killing XTOL?)
When you buy water in the store, be sure to read the label carefully. Some companies use spring water or tap water which still has minerals in it - not what you want!
"Deionized" and "distilled" should be pretty much the same thing for photography purposes. I worked in a clinical laboratory for 22 years and all of our lab water was deionized rather than actually distilled. Of course, we monitored our lab water for purity to be sure our deionizer was working properly. The few times our water got out of spec, some of our chemistry results got very ugly.