It depends on what merch exactly you are talking about. It will be for sale at various places, there isn't a generic buy all AP merch here location. IE the F&W AP pin is sometimes at the F&W Festival Headquarters, or may be at the Epcot Pin Station. Things like the AP shirt for MNSSHP (think they still do these) are available at the party merch location, but you would need to show your AP to be able to buy it. Usually whenever the merch is announced, they will tell you the location that sells that particular item.