This matches the 1 i linked on ebayI found something here https://www.performanceparts.cz/en/coolant-hyundai--original/ or https://www.mlperformance.co.uk/products/hyundai-99927adep02-ready-to-use-coolant-eco-type but I have never order from them.
Anyway not to many chances to find this stuff online. The code is 99927ADEP02. I suppose you are from UK so try search that code on some relay-able parts stores from your area.
Your probably going to laugh at me now but my i20N is running slightly below the minimum when up to temperature so I think it needs a top.Simply use OAT as G30 / G12+ to fill it up.
There is also n adequate G30 ready-mix on the market, or G12+ ready-mix.