The only place I've found Haunted Mansion merchandise is right at the exit to the ride. There is a kiosk as you leave, on your left. I've seen t shirts, souvenirs and glasses there. If you are looking for haunted mansion artwork or the more expensive collectible figurines, check out The Art of Disney in Downtown Disney. We brought home way too many haunted mansion artworks from here last time.
We've also found some artist pics and figurines at Hollywood studios, The Magic of Animation
Hope this helps.