Sour cream is available at several locations around Cascais. Aldi and Auchan are good options; look for it labeled as “nata ácido.” You can also find it at Intermarché and Continente, although some expats have noted that the texture can vary. Please note that on the Continente website, it may not be listed under crème fraîche, despite the label. Here is a link to the product on Continente’s website.
If you have trouble finding it, you can buy regular cream and add lemon juice to achieve a similar taste. Good luck, and I hope you find what you need soon!