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Kashtan's firebomb was made from an oil flask. I think it was quite effective. We don't have any other weapons with area effect. Arguably on stone and in this cold it's not optimal but imagine a combustible environment like a wooden building.
I'd go with 3 oil flasks. One for your lantern and 2 for firebombs. I have some wicks but for the price we can buy another two. Does agar allow to store firebombs indefinitely? For 1gp it's not cheap. Still 1sp per bomb. A vial for glass shards maybe.
Explosives - I'd have to think about it but I'd rather buy them either in Neverwinter or when we return. We need a sturdy container not glass for a bomb. Ideally some metal pot with a screw-on lid so the pressure builds up until it explodes. A pressure cooker is ideal but I doubt they know about it in this world. Also we are already buying more than selling, that's not the point.