Welcome.
What insulin are you using? Good insulins are the human Lantus and Levemir and the pet insulins ProZinc and BCP PZI. For those two human insulin it is best to get the 5 pack of 3 ml disposable pens via a 10 ml vial. Although per ml the vial is less expensive most cats will not use up a 10 ml vial before the insulin goes bad/becomes ineffective. The human insulin N/NPH is sometimes prescribed but only lasts 8-10 hours. Same for the pet insulin Vetsulin/Caninisulin.
Many of us here order Lantus and Levemir from Canada
http://www.rxcanada4less.com/search.php
Much less cost than in USA
Most of us here test our cat's blood glucose at home using a human meter. We test before each shot and periodically between shots. We record our reading and other info in a spreadsheet. See:
http://www.felinediabetes.com/FDMB/threads/fdmb-spreadsheet-instructions.130337/What are you feeding? A low-carb canned is best. No reason for a prescription food. Here is a list of commercial low-carb canned
http://www.felinediabetes.com/FDMB/threads/shortcut-shopping-list-all-8-or-less-updated.117688/