A new brand to explore, always nice! Especially when it's as promising as KWZ Ink (KWZI), a young, Poland-based brand ran by Konrad, a fellow chemist (+1 for fellow chemist buddies!). It's somewhat hard to come by their inks, apart from a handful of stores that offer them. Of course Konrad has a website of his own where you can order them directly, but it's not a full-on store. I didn't actually know of any nearby stores that had these inks for sale, until I visited Sakura Fountain Pen Gallery in Diest (Belgium) a couple of weeks ago! This relatively new B&M store, located just a short trip from my home (yay!), offers a rather complete range of inks from various brands, of course including those from KWZI.
If you want a bottle, a quick google search will lead the way towards the few online stores that sell it. If you happen to live in Europe (or like to buy overseas), I'd recommend checking out Sakura Fountain Pen Gallery, they also have a nice assortment of other less frequent ink brands available! Prices for the KWZI iron gall inks are 15 EUR/ 14 USD, the 'normal' inks go for slightly less.