The "old fashioned" way was to have one of these (I'm sure an equivalent can still be found), fill it with water and give it a shot of CO
2 from a single dose cartridge cylinder (readily available) - I always remember my father doing this.
If I ever need soda water, I just use ordinary fizzy bottled water - heathen that I am (isn't that what it is, anyway - which is maybe why you're having trouble finding it).