You really don't need the pressurized cyclinder to make them at home. That was more of a candy store/luncheonette thing.

I make them at home WITHOUT the pressurized Seltzer, instaed just pouring the seltzer in as I stir the syrup and the milk. It doesn't have exactly the same effect, but it comes out pretty close.

Years ago many people had a "Soda Man" who used to deliver Hoffman's and Dr. Browns soda directly to your home. One of the perks of having soda delivered to your home was that the Soda Man would provide a Seltzer Bottle with a pressurized cylinder and would fill it up for you on a need basis and also replace the bottle if the cylinder stopped working.