https://www.vinbudin.is/english/home.aspx
Google Maps Reykjavik
It's good to keep in mind that alcohol, wine, spirits and beer is only sold in special stores owed by the government, one is in 10 walk from the penthouse in Reykjavik and 10 min drive to Hella from the cottage.
Alcohol is expensive in Iceland due to high taxes but it can be bought tax free in the Tax free store at the airport which is placed next to baggage claim area at the airport.