You should be able to buy GSM modems from mobile phone/cell phone shops, else places like eBay or the mobile phone operators websites themselves. You might also find them for sale in computer shops. If you search for or ask for the actual name of one of the modems listed in "Requirements" you should have more luck. Just make sure that anything you buy online is "unlocked" so that you'll be able to use it with a SIM card from one of your local Belize networks.
There are a few phones also listed here, so if you really have trouble finding a modem you could always try a phone (although these are not recommended).
Unfortunately we cannot help with finding devices, so I'll keep my fingers crossed for you!
P.S. If you want a bit more information about how FrontlineSMS works, and GSM modems, watch the introductory video here: http://frontlinesms.ning.com/video/2052630:Video:17509