All the major supermarkets are in White Sands Beach. There is Big C, Tesco and Macro. I prefer Macro, it is the largest of them all and has the best selection of food and drink. It even has quite a selection of strong alcohol, including western brands.
It also has a great selection of fruits, vegetables, mushrooms, seafood, chicken and other meat. Just remember that things are to be weighed in the section they are found at. For example in the produce section there will be a clerk in the middle with scales who adds the price tag to the bag of veggies you just picked out. Same thing with the seafood section. There is a weigh station and a clerk. The same with the meat section, although that station is more obvious as it is behind the row of meats which you pick out yourself by putting on a disposable glove and put the amount of meat you want in the bag, and then hand over the bag to the clerk to have it weighed and tagged.
Trust me, it is embarrassing to come to the cashier only for them to realize that you forgot to get the price tag on some product and now you are holding up the line, and someone goes running to the section to get it tagged etc. Just double check with the clerks in that section if anything needs a price tag.
However, Big C does have a better selection of appliances. If you have a fridge, you can have bread, butter and cheese at home and buy a single grilled sandwich any time you want for a low low price of only 300 baht. And if you are doing a longstay, you start to get comfortable and cheap. Why pay 30 baht for a grilled cheese at the local café, when you could make them at home for 10 baht each. And now you may be thinking yeah, but it’s 20 baht.. Who cares? Which I can in a way understand. But on the other hand. It is also 4 chicken skrewers. So basically a snack or a very small meal for the difference.