I think PoserWorld has some. Not sure though. Which kind of costume? Baby clothes or halloween stuff?

If it's the latter... do you have Texture Converter? I once made some halloween textures for the Mill kids (first version. They take V2 textures - and Mill Baby takes V3 textures, so a conversion should be possible. If that's what you're looking for, they're on my page under textures :) (I recently found my long lost zips for them, they've been offline for years, but they're finally back :) )

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.