Ebay. I buy most of my little girls' hair accessories from there.
Searching hints...
Under "categories", select the "clothing, shoes, and accessories" subcategory. Then select the "girls" section.
Now do a search for the type of item you are looking for. I usually search for Gymboree korker hair clips, so my search might look like:
+gymboree +korker
If I were searching for a specific line, I would add that in as well, for example, from the recent "new york girl" line:
+gymboree +korker +"new york girl"
You might want a headband, maybe primarily pink:
+headband +pink
I find that Ebay can be kind of picky about their search terms, if i just search for gymboree korker without the "+" signs, I get about a million results because it shows me anything with either gymboree OR korker in the listing, when what I want is both.
Also, if you are looking for a hair accessory for a certin line of clothing, but can't find it, for example Gap's "Treehouse" line of kids' clothes, you might try +M2M +gap +treehouse. "M2M" means "made to match", and it will show you all the accessories people have handcrafted to match certain clothes. M2MG is generally accepted to mean "made to match Gymboree".
Have fun!