The shop in La Molina is Gemma which is about a 6 minute walk. It is open 8am to 8pm most days, but reduced hours in the off season.
There is a bigger supermarket in Alp which is open to around 8:30pm Monday to Saturday. Its about 15 minutes drive away.
The best shop for most things is Carrefour in Puigcerda this is open 10-9:30pm every day including holidays. Its about a 20 minute drive away.