Do you have a main treating physician for your diabetes? Most physicians receive free samples and if you let them know you need help they will give you a months worth.
This site has some links to government aide that is available for Diabetes Patients:
Financial Help for Diabetes Care
I searched using diabetes help paying for insulin and a lot of information and message boards with people asking similar questions and getting responses on what they did to get free or reasonably priced supplies.
Your rehab should cover this as it is integral in learning to live a sober lifestyle so I would maybe take this one day at a time and make sure that your treatment facility knows that you need assistance in getting coverage. How long is your program?