In New York I've never had any problem finding seltzer. My parents buy it every time they go to the supermarket in upstate New York.
I'm trying to think of how common it is in corner stores. I can't say I've ever looked particularly hard for it. I used to keep my refrigerator in Buffalo stocked with the stuff though. It sure is refreshing.
Thankfully my work provides free cans of seltzer which I drink like water (heh heh!). Although I occasionally wish they would get the kind with a citrus fruit flavoring instead of just plain.
When my parents took us on holiday to America my little brother refused to drink either plain water or fizzy drinks, but we found orange squash in the British section of a large supermarket.
As Wikipedia says, it's the default drink for British children. Adults who drink it probably pick a nicer product; my manager keeps a bottle of elderflower cordial in the cupboard.
I don't like making generalisations about Europe, but carbonated water is sold in every shop in Europe that sells bottled water. In some countries (e.g. Georgia), in smaller shops, you will only find carbonated water. In Germany it's what you get if you order "mineral water", you have to specify "still mineral water".
And restaurants in Europe can't seem to give you just plain water unless you specify. I spent my entire vacation being surprised by bubbles before getting what I wanted.
This is entirely psychological, right? What about soda would make it more refreshing physically than water?