Serbia has lots of mineral waters, with pretty good composition, and many of them are actually a double operation - from the same spring part of the water goes into bottles, part into the local spa. Few of those spas were established back in, who knows, maybe even roman times. The current spa culture has at least a century or two of tradition, the locals are quite used to have guests in this sort of internal medical tourism. Off the top of my head, „Knjaz Miloš“ from Bukovička Banja (banja is from italian bagno, a spa), „Vrnjci“ from Vrnjačka Banja. Sokobanja doesn't bottle its water, as it's more for bathing.

There being a score of brands from as many springs, as a result this is quite cheap, and many of those are sold by sixpacks - easier to get into and out of a car trunk than individual bottles. The size of the bottle is, as a rule, 2 liters, except Knjaz which is 1,75.

There's a special dispute whether mineral water is permissible in a špricer.

Note that hardly anyone says „mineralna voda“. The word is „kisela [voda]“, i.e. sour.