In September and October, the slowest months of the year in the western Caribbean, you can find plenty of room in San Pedro and elsewhere in Belize. You will not be left out on the beach without a room over your head -- I personally guarantee it! In this slow season, a number of hotels are closed, though not on San Pedro, because it is so slow. Walk-up rates ARE available, though hotels don't particularly want you to know that.
Having said that, you can probably get almost as good a deal by booking direct in advance on the Internet. There are some really great deals now in San Pedro. So you don't have to worry you might go ahead and e-mail your top choices and ask what is available.
Belize First Magazine http://www.turq.com/belizefirst/