I'd love to try this myself, but where I currently live - despite being an ingredient in a few traditional cured meat dishes - nitrate salts require a virtually unobtainable (to a private person) permit to obtain, because of terrorism concerns.
An other option is ammonium nitrate at +25.69, though you might also have trouble with terrorism regulations as it's both a common fertiliser and a component for explosives.
Ammonium nitrate used to be uses in the "instant ice/cold packs" for first aid, but has since been replaced with urea - I believe due to the terrorism regulations.
Which means (as another poster suggested) I'll be trying urea as a way to chill drinks sometime soon I guess.
Not surprised by your story, but still curious where you live? In the US, you can buy it in bulk quantity on Amazon. Mixing it with sugar was a ton of fun when I was a kid, smoke bombs that could totally white out a large backyard.
I could easily order it from overseas online, but the consequences are pretty harsh (violates the explosives laws). Not worth it tbh.
Somewhere else I lived in Eastern Europe, it was available in shops in 100g bags for curing meat, etc, or at the garden shop in 25kg sacks as a fertiliser.
It used to be ammonium nitrate, but most brands switched it to urea a few years ago - presumably for reasons of "people could use the extremely nice, uncoated, very pure ammonium nitrate for nefarious purposes" (useful for explosives and methamphetamine manufacturing!)