There are no up-to-date bus schedules on the web. There is one that Tony Rath put up, but it is four or five years old and useless as a schedule now, since most of the bus companies listed on the site no longer exist.
Bus schedules change constantly. If you go to the Novelo's terminal in Belize City, you will usually find fairly current schedules posted there, and Novelo's (and least at the current moment, though this may not continue too long) and its subsidiaries are the main bus company in Belize now.
--Lan Sluder