Batgirl is currently expected to be a HBO Max exclusive. You can use this link to sign up for the streaming service.
In the comics there’s been countless times that Batgirl and Harley Quinn have interacted, both as foes and allies. So a crossover could be fairly easy to pull off, as long as Leslie Grace’s Barbara Gordon is in the main DCEU timeline. Perhaps Batgirl could be the “antagonist” of a Gotham City Sirens movie, as she tries to track down the likes of Harley, Poison Ivy, and Catwoman.