I'm not down in Placencia, but I think the info below is pretty accurate .........
Belmopan has a farmer's market every week - fresh produce, good prices.
Fish comes from local fishermen with whom you need to cultivate a relationship.
Contact the producers of meat and poultry to see what they do for orders etc.
Most other wholesalers are in Belize City.
You may wish to check with Kitty Fox at Peckish (specialty foods) in Placencia as well.