Much of the Batgirl series has been traded. There are a few issues missing here and there, and some that were part of crossovers were printed in those crossover trades, but most of it's available. "Silent Running," "A Knight Alone," "Kicking Assassins," and "Destruction's Daughter" are a few that come to mind, as well as the Batgirl/Robin crossover "Fresh Blood."
But please don't get "Batgirl: Redemption" by Adam Beechen, he's the writer who sent Cass off the rails in the first place, and that mini made it even worse.