Several of the breeders will probably post roughly what they bringing a week or so before the show. In the past it has appeared in the Classifieds forum. You can make any request of a breeder re: pre-selling frogs. You may want to consider walking the show floor and seeing what is available. Seeing a frog before you buy it is always preferable to buying sight-unseen. Walking the show floor also lets you get a feel of where supply is and what the going prices are.
The only way to know what 80% of the show floor will have is to walk it and see.
Best prices? Its a delicate balance between time and quality frogs. The longer you wait the greater the chance it will be bought by someone else. Know what certain frogs are selling for BEFORE you go to a show. If the price is right - buy it if you like it.
Some frogs are "hotter" at some shows - more in demand. Its hard to know in advance what those will be. They will sell-out very quickly.