Julie,
Many prescription medicines are available in Belize, at least in Belize City and in larger towns. I don't know if your particular medicines are or not; if they are not commonly prescribed, I would not count on it. Your foreign medical insurance definitely will NOT pay for any medicines or any kind of care in Belize, unless it is a special policy written just for expats.
Since you are going to be in Corozal Town, you might want to hop over to Chetumal. San Francisco or another store may have your medicines at fairly low cost.
--Lan Sluder
<author of
Adapter Kit: Belize, Avalon Publishing, 262 pages, many maps and photos, $17.95, the first and only comprehensive guide to living, retiring, working and investing in Belize>