Hmm I was told Panama BUT what I found may have been cheaper but often was poor quality. E.g. Rice and t-bags ... buy what you like now while you can and complete the provisioning in Panama. Of our Panama provisions pasta, Panamanian ground coffee, dried beans and tinned tomatoes came out tops. T-bags, rice and cereals were awful. Bread flour, actually U SA produced was ok. Beer cheap in Panama but quality dubious but then again by the time you reach Tonga you'll drink anything!
PS I' Ve 5lbs of brown rice useful only as ballast if you want it.... currently in Indonesia!
Leader Price in Martinique is the best place to do a major food shop. If you go to Marin the outlet there it is on the waterfront with it's own dinghy dock and you can wheel the shopping trolly out to your dink. Good selection and most items are substantially cheaper than anywhere else.
I also find that flour rice and pasta bought there and immediately double bagged in heavy duty ziplocks is less likely to develop bug infestations.
for all dried foods it is useful to deepfreeeze the bags for 48 hours if possible. This really does reduce the chances of bringing unwanted livestock on board.
sincerely i can't tell you where but i can recommend you a few useful Foods:
Canned Alaskan Wild Salmon
Brown Rice
Dried Beans
Bulk Nuts
Peanut Butter
Energy Bars and Chocolate Bars
Dried Meat / Jerky