We got most of our seedlings (mostly fruit) from Springfield Nursery. Dave Harder is really knowledgeable, and can advise on your soil conditions, drainage, shade/sun etc. The 2016 pricelist is on the website.
Between Belmopan and San Ignacio, we like Riverwalk Nursery. Not big on fruit trees or berry shrubs, but lots of ornamentals. Lovina Swift is also excellent to help you choose what could go where. Riverwalk Nursery has a FB page.