I'm doing a cosplay!

Twilight sparkle : try to do a nerd/ bookworm outfit. Like get a sweater vest and dye it twilight Purple then a white collar shirt also plad skirt would go well.
Flutter shy: I've seen great Flutter shys with green airy dresses that incorporate alot of nature, also if she caried a basket with a plushie of angel that would be great.
Rainbow dash :she needs like a light blue warm up suit that you can add her cutie mark to, or something really sporty.
Apple jack: she just needs a form fitting flannel shirt with a complimenting t-shirt under it and jeans with boots.
Pinkie pie: like a fun short party dress with ruffles at the bottom and of course it's all shades of pink.
Rarity: an elegant ball gown would be great that is white with long gloves and maby a jewel necklace
Apple bloom: try to get cute overalls and try to keep with the apple jack concept
Sweetie belle: maby like a white sun dress and flats
Scootaloo: like rainbow dash very sporty maby knee pads and a raged old t-shirt, and jeans.

Try the costumer. Com for the wigs they might have the style you need. The hope this helps