You can buy fruits and vegetables at the supermarkets which are all over Venice. But it is fun to go to the Rialto Market for vegetables and for fish. We shopped there when we were in Venice for a month and patronized particular vendors so they came to recognize us. You will need to use some Italian---express the weight of veggies you want in "etti" which are 100 gram increments. "Due etti" would be 200 grams or a bit under half a pound. For fish, you buy some by the whole fish, and some, like tuna or swordfish, by the piece---you can point at the one you want.
For supermarkets, we like the Punto Supermercato in San Marco as it has wider aisles than some, and a good selection. This would be on your way to the Rialto Market from Dorsoduro if you go via the Accademia Bridge. Cross that, continue straight on through Campo San Stefano and C. San Angelo. The market is to the left on Rio San Luca before you arrive at CamponManin, but you may have to go into the campo and cross back over the canal to get to the front door.
There is also a Punto in Dorsoduro on Campo Santa Margherita which may be more convenient but we have not been in that one. And at the end of the Zattere, toward the cruise ship dock, is a Conad supermarket. This used to be a Billa and when we were last there the aisles were very small and the place very crowded. But it may have been remodeled. Also avoid going in late afternoon which seems the busiest time.
There are also Coop supermarkets around, two in San Polo that I can think of. We used these on our last visit but again, one ( near Ponte dei Meloni) was very small and crowded ( and hard to find), and the other, on C. San Giacomo dall'Orio, quite nice. This one would be out of your way but the campo is my favorite in Venice and worth a look. They had a little neighborhood festival going on the Saturday we were there---live music, buskers, flea market, political booths, etc. I don't know if this is a regular thing and winter might not be the right time.