Could you post some references of which costume of hers you're working on? That would help a lot.
If you're doing the two-tone bodysuit, your best bet is going to be some type of spandex. You can probably find a decent selection at your local fabric store (Joanns or Hancock), or anything else online.
STL Comic Con 2014 | C2E2 2014 | Heroescon 2014
Costumes: Miss America Chavez, Young Avengers COMPLETE
Peggy Carter, Captain America: The First Avenger COMPLETE
Pin-up Carol Danvers as Captain Marvel, Captain Marvel COMPLETE
Fem!Steve Rogers, The Avengers 50% DONE
Planned: Sif, Journey into Mystery, Thor | Dahl, Riddick | damsel!Valkyrie, Fearless Defenders (#12 cover) | Giganta, DCAU