Do they ever retire, or are they available as a continuous item?

If you're asking about the subeautique, they retire. If you check here you can see all the past and current sets.

If you're asking about the boutique in the cash shop, I believe those also retire. There was a set that just retired recently , if you check the last few pages of the news you'll likely see it.

They do retire after awhile. You can grab some pieces (if you're lucky) with the CS Hustler. Sometimes the staff bring them back for a limited time too.

Ahh I see. Thanks for the info, guys :)

You can get retired Subeautique items from Gift of Fashions Past as well. It gives you 10 items when opened. The items are random, however, and it also gives retired costume trunk items. If you're after specific items, you're better off buying them from other users.

Wow, that's handy to know. :) Thanks.