Have you tried Belize City you can pretty much get anything (well almost) If you head to the bridge by the boat ferry terminal and go over the bridge theres a pharmacy right in front of you and they have a good stock on most things. San Pedro may not have the choices that Belize has.
Or why not get them shipped to you, you have a prescription so you are covered by the law so maybe contact whoever gave you the prescription and have them send you a new supply.
Belize Medical Associates
5791 St. Thomas Street, Kings Park
P.O. Box 1008
Belize City, Belize C.A.
They say they have a good pharmacy and if they don't have it they can order it for you.