The closest possibility to buy food is in Hella, which is a 25 minutes drive away. There is a grocery store (Kjarval) and a petrol station. The petrol station is open until 23.00 every day.
If you want to get alcohol there’s only one store chain in Iceland. It is called vínbúðin and is closed on Sundays and holidays and also closes at 18:00 every other day. The closest of these stores is in Hella.