There are Disney outlet type of stores that sell old park merchandise but marked down. They can be found in both of the Premium outlets at each end of the i-trolley route.
The ones at the premium outlets are great they had some really good stuff at great prices. I got a really cute top for about half the price than in the parks. DH got some shirts as well.
They had current halloween merchendise at less than the parks and 2010 xmas baubles (so current stock).
Clothes wise is was mainly very big or small sizes - DH got a grumpy dwarf hoody for $10.
Keep your eyes out in the parks. Halloween stuff was reduced a few days before (after i'd bought what i wanted!).
When we were there in Dec 09 if you ate at some cs places you got a coupon for a percentage off merchendise (10%?) before 12 noon. Not sure how often this runs.