I got mine at walgreens, but it is really hard to find. Ask for help if you don't see it. Any pharmacy should have it, or you can get it online.
Yes, since she is new at laying, hopefully she is just getting the kinks out.
You probably want to have a bottle of calcium citrate with D3 on hand for her if she is having trouble. That can help them pass difficult eggs, and gives them a short term boost of calcium.
You can also try flaked oyster shell to see if she'll eat that one more readily, but everyday layers will...