Have you tried contacting Diabetic Cats In Need (DCIN)? It might be worth enquiring whether they might be able to help with the cost of the insulin. Check this thread for details on how to contact them:
http://www.felinediabetes.com/FDMB/threads/financial-help-links.131190/
If you haven't already done so I strongly advise you to pick up some ketone test strips (I think Walmart may do reasonably-priced ones) so that you can monitor urine for ketones. For general information (just to inform, not to scare!

) if you ever get a trace ketones result you need to contact the vet for advice on how to proceed. If ever you get a result more than trace ketones you need to seek immediate veterinary treatment in order to prevent the kitty from going into diabetic ketoacidosis, a life-threatening complication of diabetes. It's hard on the cat and very expensive to treat. Far better prevention than cure.
Mogs
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