As for supermarkets I used to do my shopping at the BM store, which is basically a small Walmart. For veggies, fruits and eggs you should definitely check out the Uvita Farmers market (https://www.facebook.com/elmercadocr/) and it is located just a couple hundred Meters down the street of the “French Café”. You can experience the Tico culture and get to know local businesses every Wednesday & Saturday morning from 8am to 1pm.