I would definitely recomend the Grand slam Pitching machine, we have used several different ones, but this one is consistent, and you can change the speeds by changing the amount of air flow to the balls pitched. Also, you can use the typical wiffle golf balls or the hollow ones.
We use ours at every practice. The girls got frustrated the first time or two they use ours, but now they love it, and I can honestly say we have hit the ball a lot more consistently this year than last.
Here's the link:
http://www.agrandslam.com/
The long tube, helps a ton with consistency and staying with in the strike zone.