I usually drive to Lamanai so I have not gone with them personally, but I hear good things about the Novelo brothers who do trips from Orange Walk. Believe they operate out of the Lover's restaurant in Orange Walk Town. Several other operators do trips leaving from the New River/Tower Hill bridge south of Orange Walk. Once at the site, if on your own, Nazario Ku, who is the head caretaker and a guide, is great.
--Lan Sluder