so this may be--and it is, actually--a silly question, but is the pixie/fairy trunk retired?

it was one of the original costume trunks, but since remaking it, does that make it available?

Yep, it's available from the MCT, as one of the options.

Well the Fairy Costume turns into Box of Fairy Dust when used. And I believe there was an item that was automatically turned into the Box of Fairy Dust. And you can buy it from the MCT as well.