I'm visiting Iceland this august, will be arriving in Reykjavik. Since it's not possible to bring gas cartridges with me, I'll need to buy them there. I'll be arriving on a sunday. Is it possible to buy gas cartridges like here somewhere in Reykjavik on a sunday, or are all stores closed on sundays?
I Googled and found there is a least one store (top return) that is open daily from 9AM to 5PM. It also offers 24/7 pickup and drop off of rental supplies. So yes you can get gas cartridges on a Sunday in Reykjavik.
Their address is as follows:
101 Reykjavik - Iceland
Tel. +354 647 0569
On my last trip to Iceland I got the tip to look around at the air port for gas cartridges. Often people who are leaving drop their stuff there.
Also hostels might be a good place to search for. The biggest selection I probably saw, was at the campsite in Reykjavik.