There isn't any way that you would be able to access a master list of everything that is sold anywhere in DW. Can you imagine how huge the inventory list of all items would be? And as mesaboy pointed out, it would be in a constant state of change. You will have to look in every gift shop, stand and kiosk to see what is available when you're there.
You'll find that many stores carry the same items, but there are places with a good assortment of unique merchandise. Gift shops at the end of a ride often carry items specific to that attraction. Items specific to a park (e.g. something that says Epcot on it is not likely to be in another park, but you might find it at a nearby resort or DTD.) There were many items in AK/AKL that we didn't see elsewhere. World Showcase has more unique items than anywhere else in DW. Some of the resorts have resort specific items so don't overlook the giftshops in the resorts. One thing to keep in mind is if an item is only sold in one or only a few places the inventory may be limited and may not be replenished once it's sold out.
