I buy distilled or demineralized water at the local pharmacy, in gallon size. I think it is more to prevent scale formation in some appliances. Anyway it works well.
You should not use soap for final rinse of your negatives. Instead, use photoflo or similar product meant for this specific purpose.
There really should be no need for distilled water for mixing regular chemicals like fixer and D76
I know that I'm in a different country and another continent but I use de-ionised water from the local auto spares shop, it's a lot cheaper than distilled over here.
Ebay has what is called a TDS (total disolved solids) pen ... its 22 bucks.... there cheap... but well worth the money...... by the way some distilled water sometimes minerals for taste because it is used for baby food formula ect... tru distilled water is hard to find.... i use walmart water ....cheap and has 80 TDS reading....real distilled water has a TDS under 20.....