Prefab coops aren't usually well made (and very expensive). The best thing would to make one or get a local carpenter who is willing to make it.
Personally I couldn't make one so I got the Omlet Eglu Cube coop with run. It was expensive but my 6 chickens love it and it is very easy to move...
Sav-a-chick is good.
I would feed scrambled eggs (for protien) and make sure it's eating and drinking properly.
It could just be a sprain or a vitamin deficiency.