To find a costume for the party on Poptropica go to super hero island and get a super hero costume then go back to great pumpkin island at night(in the game) and go to the pinkhouse on the right side.
after you get past the old house on the snoopy game
After you helped Snoopy get across "enemy grounds," get the mask on the scarecrow and proceed to the pink house on the left.
After playing War with Snoopy, you'll find a scarecrow wearing a green mask. Jump up to get the mask and then enter the party.
you get Nessie follower and bigfoot costume
You can find it at party city
party city
after you get past the old house on the snoopy game, get the mask of the scarecrow, and go into the next building.
The "Zombify" card was added as a costume in the party after you beat the House. (see related question)
no you can not.
in the store
The costume, like others at the Poptropica Store, has an effect built in, but will not change the play of any Poptropica island or game.
You can find a cat costume for a two year old at many local Halloween costume and party stores. Stores such as Party City, Michaels, Walmart, and Target also offer them.