Oh, man, you are lucky! I desperately want to do a group costume (I'd want to be Rainbow Dash or Applejack), but alas, I have no real-life friends with whom to do it. I've got one full-fledged brony buddy, but he's...kind of a big guy. Maybe he could be the Big Mac to my Applejack or something. XDD If I can't do ponies, though, I have a lot of other Halloween ideas...might do a Facebook poll next month and let my friends help me decide what to go with.
Anyway, I would try simply image-searching for "light pink" or "pale pink" or "baby pink" wigs online, and when you see one that looks Fluttershy-like, click to see which store(s) sells it. There are a couple of mega costume shop sites that would probably have the greatest wig selection.
For instance:
link
I see some that could work with a bit of styling. :D
Anyway, I would try simply image-searching for "light pink" or "pale pink" or "baby pink" wigs online, and when you see one that looks Fluttershy-like, click to see which store(s) sells it. There are a couple of mega costume shop sites that would probably have the greatest wig selection.
For instance:
link
I see some that could work with a bit of styling. :D