In Arinsal there are two supermarkets. St Moritz supermarket is located in the St Moritz Appartments building, and Moles is located in the Solana Hotel building.
Both of them have a big variety of products and of course fruit and vegetables.
If you don't find what you want in Arinsal, you could always go to La Massana where you can also find several supermarkets slightly bigger.
Please, do not hesitate to contact me in case you have any question,