The closest mini-market is the Coop in Castellina Scalo (7 min by car from Stomennano). Open Mon-Sat 8:30-1 pm and 4-7:30 pm. Closed Sundays. https://goo.gl/maps/h688GMgRuNaxUwRz7 The best full-stocked supermarket is Conad in Colle Val d'Elsa (15 min by car from Stomennano). Open Mon-Sat 7:30am-8:30 pm and Sundays 8am-8pm. They also have a really nice beer, wine & spirit selection. All of the rooms on property have a kitchen so we suggest stopping by at some point to grab anything you might want in between meals (snacks, alcohol, etc.) https://goo.gl/maps/Loi8hSZFMMabP6jh7
There are no CVS or Rite-Aids in Italy.... pharmacies basically sell only medicine and things related to personal hygiene. The closest pharmacy (7 min by car from Stomennano) is open Mon-Sat 8:30-1pm & 3:30-8pm.
If you forgot your swimsuit, yoga pants, or sunglasses, there is a Decathlon nearby (10 min by car) for all your sporting goods needs. Open daily from 9am to 8pm. https://goo.gl/maps/8FuWZB3Z4WBSp5wY9