One of the main problems with getting flies from say Kaufmans or The Fly Shop is that those flies usually come from Umpqua Feather Merchants and you miss out on the "newest" or "local" flies being used by us. I don't mean to make it sound more complicated than it is,..... but it is. You can probably relate based on what flies you use in Montana compared to the trout flies that may be sold by a Saltwater fly shop here in Florida.
I can tell you that the flies I use here in Tampa Bay arn't what you will see in Kaufman's and the same goes for what I use in The Keys.
If I were going to Belize, I would call the Fly Shop in Redding, CA., they use to do alot of trips there. For the Florida Keys, call Sandy Morett's in Islamorada and Dan's Custom Flies for the West Coast of Florida.
Also you may want to get on-line and get copies of the two saltwater flyfishing magazines as well as the new Shallow Water Angling magazine by Florida Sportsman.
Good luck, let me know if I can help anymore, Mel