Did you call other veterinary clinics in your area to see if they have the med, how about the emergency place, they should be fully stocked?
Ask your vet about veterinary pharmacies, some are okay. Your vet can fax them a prescription, the vet may ask you to sign a waiver.
I have used that California Pet Pharmacy in the past without problems for heartworm pills.
https://www.avma.org/public/PetCare/Pages/online-pharmacies.aspx
PS: Is she due for an annual exam? Maybe the vet wants to re-evaluate the dog first, regarding the need and dosage of the med before refilling. Check labs, etc.
