As others have pointed out, the Mansion is woefully short on merchandising. It's so silly, because of all the attractions that deserve a kick-butt gift shop and unique merchandise, the Mansion is it.
At the cart outside the ride, you will find mostly (unrelated, grrr) Nightmare Before Christmas garbage, a CD soundtrack, the Haunted Mansion book (sometimes, same one you can get at home), and I believe they often have the villians beach towel.
However, on my last trip they had added something, and it's proudly in my living room right now - a faux license plate in black, with the Haunted Mansion logo and DOOM BUGY (sic) as the letters. It also has the Magic Kingdom logo on it. I have it up on the wall. It's really the only "unique" item they have, and I really enjoy it. Plus, I think it was like $7, not bad for a Disney souvineer you can display.
AEfx