It's been a couple of years, so I don't know if it is still there. However, there was a great shop in Belize owned by an American ex-patriate who moved there with her family from Atlanta. Anyway, she represented many of the local artists and craftsmen. Her prices were reasonable and the stock was lovely. Plus, she was a lovely woman and we spent as much time chit-chatting with her as actually buying things.
Anyway, her shop is a white building to the left just as you exit the pier area (you will go through some gates). I hope she is still there, because I would love to go back!
There is a National Handicraft shop in Belize that is supposed to have Mayan handicrafts at very good prices. I have not been there yet but I plan to check it out when I am there 1 month from today. I hope this helps!