We will be chartering a sailboat from Placenciar. Are there grocery stores where we can do our own provisioning?

No problem getting the basics in Placincia. Wallens grocery is probly the biggest and would have most everything you would need. There are several smaller grocers as well. HOWEVER, If there is something "special" that you just cant do with out,you would be well advised to bring it with. There isnt a Safeway or Costco in sight (thank gawd)

Having spent 4 months down in Placencia over the last 2 years I can tell you that you can't count on what you NEED being available.

Better to have the charter company do your provisioning for you if you're starting from Placencia. Even basic items are frequently missing from the shelves (milk, veg, fuits). Your charter company will have most items flown in from Belize City. It's worth the cost and saves you the hassle of hunting for everything and will get you out sailing MUCH faster.