My grandmother lived in Coney Island until she entered a nursing home around 1993. She always had the old-style seltzer bottles delivered, so I guess there was at least one company that still provided it back then. I'm not sure about nowadays, though. Maybe someone on the Boards knows more about this.
I doubt there are more than a handful of guys still making seltzer deliveries in the City (if even that many).
My family moved to Sheepshead Bay in 1955; there was a regular seltzer guy who handled our co-op. There were 240 apartments there and almost everyone had seltzer delivered once a week (including my family)... he'd also deliver Fox's UBet syrup (the large bottles) and those heavy seltzer bottles (with the spritzer on top) made making an egg cream absolute joy. He delivered there until around 1970 or so, but by that time most of the kids had grown and I guess the parents didn't want the seltzer delivered anymore.
Still, it used to be funny to walk down the hallways (in the 50's) and see 5 or 6 seltzer bottles in front of everyone's door.
