I think she got the syringe when she was visiting Cindy at work. She probably just reached into one of those red toxic waste disposal boxes that are all around medical buildings.
That could be as good an answer as all the others that have been posted. I'm not really sure where she came up with it, but with Casey where there was a will there was a way. I don't see her being unable to accomplish any of the things that we read yesturday in the doc dumps. Some really sickening stuff but I read it all anyway. I wonder when it will dawn on Casey that she is never leaving that jail as a free person to live in society and have the freedoms we all do. I'm waiting for that day.