I personally love http://www.dollclothessuperstore.com/mam.html for Barbies and doll clothes and you can get her a matching outfit. Not sure if they'll ship in time for Christmas, but you can ask. The owner is supernice and always upfront with you. I got some great Ken clothes there for my SD a while back.
If she's into cooking and you are near an IKEA or can get it shipped, look there for their play food and pots and pans. Yes, she's 6, but they make great "real" products that many kids love for years.
Would she like her own bath stuff? A robe, a body poof? Maybe get some Bath and Bodyworks stuff. I think they sell American Girl branded stuff.
What about books? I'm a big fan of books for kids. Does she like jokes? Amelia Bedelia might be a good series. Or start reading her Little House on the Prairie or Ramona.
An art kit is good. Michael's sells several different sized kits, and I think both they and IKEA sell small easels and paper.
Sometimes the best gifts aren't requested. My SD asked for one of my flash drives last night. I am gifting her one for Christmas. I have enough Staples rewards that I think I can get her 8G for nearly free. Keep your ears open to what she talks about.