there are a few designer shops near the harbour in ibiza town. more your boutique thing than big name stores.
if you like lacey white dresses though, you are in luck: ther's millions of shops selling 'um
If u head into Eivissa town u'll run into a lot of shops that carry designer clothes. I found these 2 really cool shops and later found out that they were owned by the same ppl...hahahaha crazzyyyyyyyy. Look for boutiques called Vestium, and Bad Boy Ibiza...they carry Jean Paul Gaulthier, Armani, D&G, and a whole bunch of Energie lines...hope it helps