Savannah's doing my work for me.
There are several different brands of liquid calcium so I don't want to suggest a particular brand that you might not be able to get. Liquid calcium is often sold behind the counter in the drug store, but sometimes you can find in on the shelves in the vitamin area.
If she's still eating stick with the plain powdered calcium on her food. Calcium carbonate (the powdered type of calcium) is better for them in the long run. The liquid calcium has two or three different forms of calcium that are easily absorbed but that apparently don't build up the bone as well as calcium carbonate does. (can't remember my source on that- maybe Mader)