When searching for primary sources, note that Belize was renamed from British Honduras in 1973 and was part of the British Empire until 1981.
The British Library contains materials related to and from Belize, including the digitized newspaper Colonial Guardian (1850-1899) held at the British Newspaper Archive.
The Library of Congress has digitized materials related to Belize, including maps, photographs, and drawings.
The UK National Archives contains digitized materials related to Belize, including wills, government and military documents, and ship registries.