I've never really looked for gas in shops in towns, but a quick google shows Clas Ohlson which you can certainly find in Stockholm and major towns/cities sell gas in their Swedish stores.
https://www.clasohlson.com/se/view/COSt ... reLocator/You'll also find it in some but not all country petrol stations - there tend not to be too may shops etc outside main population areas, so many petrol stations have little supermarkets which are full of useful things . They can be quite good for a coffee and snack (danish pastry, cinnamon bun etc), normally on a deal as Swedes love their "Fika" (coffee/cake breaks)
Eating out can be expensive in the evening, but Monday to Friday you can normally get a "Dagens Lunch" for a reasonable price (by Swedish Standards). Just avoid buying alcohol with meals, it will bankrupt you ! Buying bottles from the state off licence isn't too bad though, if you need a daily fix, or a pick me up ! You can even get British real ale. Just beware they have limited opening hours 4pm Sat close at a weekend.