Here are some good links for managing diabetic cats (I have one):
http://fdmb-cin.blogspot.com/2013/11/lantus.html
http://catinfo.org/?link=felinediabetes
http://www.felinediabetesremission.com/
http://www.bcpvetpharm.com/products_bovine.htm
That last link is supposed to be a good supplier for PZI insulin, which incidentally, is supposed to be cheaper than lantus. They would not give me a quote over the phone without an actual script in hand.
I get the Lantus, insulin gargline, from Target or Walmart in single pens. From Target it is about $80, and it lasts about 2 months. I’ve heard you can use the drug manufacter’s discount program to get it for $25 at walmart but I haven’t tried it yet. I do get my syringes at Walmart for literally 1/3 of the price at the vets.
If she reads the links I posted, do NOT try to do a diet change while medicating the cat with insulin unless she can competently test blood glucose levels on her own. Sometimes diabetic cats can be regulated almost completely with diet change, and can go into remission due to diet change. If you then administer insulin on top of that the cat can become hypoglycemic. Cats do not die suddenly from being hyperglycemic, but can from being hypoglycemic.