The crop is on her upper chest on her right side, it will bulge out as she eats and it fills with food during the day. It will empty at night and should be flat by the morning.I haven't felt her crop before (newbie here!) - she just ate, so I think I found it, and it's firm. A little higher up than I was expecting, too...
Azygous has given some good advice. You can get Corid at Tractor Supply or most other farm supply stores. Start there and if that doesn’t work, assess again. Coccidiosis can be really painful for chickens, resulting in an hunched up appearance sometimes.