Remember that most sites here probably have more kit available than they list, or their online list might not be up to date. So asking around when you arrive is the way to go, but from these listings you can still get an idea of the range and price of the available gear.
Craigslist – lots of great deals on here.
Neil Pryde Maui – used and ex-rental Neil Pryde boards and sails
Hot Sails Maui Shop – used, prototype and closeout Hot Sails gear
Naish Maui Specials – ex rental and closeout deals.