Call some local vets. We had a lab that was diabetic and the vet we used had people donate insulin and she would give it out to people who couldn't afford it for their pets.

It definitely saved us a little in the beginning, and then we bought whatever one we needed from Walmart. Dosing a Lab is kinda a crapshoot since you can't really take blood at home, so I don't mean to sound cruel, but it's not quite as critical.