they go to cancun or merida, cancun would be closer to belize. a (nice) bus gets you to corozal, and there are boats and planes to get to San Pedro from there. sometimes there are planes from Cancun to Chetumal, then its a real short bus, just over the border.
Aerocaribe often runs the Cancun to Chetumal route, but not all the time. I took it Nov of 2001, but it was cut off soon after. might be up again for the busy season.