There are good supermarkets in Grenada, one is right across the street from Port Louis Marina. The one at Spiceland Mall is the best stocked. Dedicatedly check out the market in St George's for fresh fruits and veg, spices and seasonings. You can take the local bus to St George or Spicelsnd Mall and then easily find a taxi back.
The fish market is also a great deal $3 a pound for fresh fish.
There are a couple small supearkets on Carriacou, but not much variety.
We find the Knights on Bequia has the best price on Captain Bliegh, our favorite rum.
When anchored in Bequia you will hear a conch shell horn blow in the morning, it means the fishermen are returning and the fish market is open
Enjoy your trip, the Grenadines are a beautiful place