Other than the ones you can temporarily copy, there are free costumes awarded in most ad-games on the site. There is also a Viking Suit at 831 AD on Time Tangled island. These and the purchased costumes can be put on or taken off at any time.
There are 4 special "phone number" costumes on Nabooti Island, but be careful if you choose one, because they don't come right off! You could be stuck with some parts (e.g. Brain hat and nerd look).
(see related question)
There is some on Super Power
i don't know any codes but you can get the cards yourself at target in australia, canada or the US.
yes the hypnotic
Other than buying them, no.
There are wings on costumes but there are no wings just by themselves!
no. i wish
you have to buy membership for poptropica.
There is no way to have a bat follow you on any island. This may be an idea for the costumes at the Store.
With membership, everything's free except the membership. You can use (but not keep) any costume or effect. Some "members only" costumes go into your item bag and so even if you stop your membership, you can keep those.
It is merely decorative. None of the purchased or free costumes or effects have any effect on the islands or ad-games. (The meteor shower throws some shooting stars out for a few seconds.)
There are many websites online that offer coupon codes or printable coupons for many retailers for Halloween costumes. You can also check in-store such as Spencers.
Any membership codes that are posted would have been used, and membership codes can only be used once.