It seems you can still access the Avatar clothing available for Halloween by direct links to the items even though they aren’t listed in the shop on xbox.com nor via the Xbox.
I discovered this whilst browsing http://www.360avatar.com/ and clicking on a link to the werewolf costume and the skeleton costume and comparing the two links. The only difference was a single number so deducing the others was easy (the variable number lists the items 1 to 27).
Here is what I believe to be the full list. Click the images to be taken to the marketplace entry on xbox.com where you should be able to buy at the stated price. I haven’t bought any of them, so I can’t confirm you can actually purchase them, but it’s worth a shot if you missed your chance.
Leave a message if you have been successful.