There are shops all over that sell religious stuff, things for the
Butsudan (home alter), prayer beads, etc. Could get it there. I’ve also seen incense sticks in 100 yen shops and convenience stores. When something is widely used in society it is readily available.
You don’t pay for the water, silly, you pay for the containers. As @FlikTheBlue points out, you can take your own container to church and fill it from there. The church I grew up in had the same type of container @jnglmassiv had, a large tank, roughly the size of an institutional percolator, with a spigot.
I never thought to ask whether holy water was simply tap water that the priest blessed, or whether it had to be distilled, or come from a spring or something else.
I’ve watched the process performed. It’s simply tap water that the priest blesses.
You can buy it online now.
There are people who burn it non-ritually but I don’t know how many.