If it didn't cost a fortune in the first place and I didn't actually use it for my dog I would sell you my sentinel
They don't make interceptor anymore.
Perhaps Heartguard? I'm not sure what the active drug is that you're looking for to kill the bugs. Does the added med in sentinel for fleas affect the reef In a negative way since everyone used to use interceptor and that doesn't have the flea portion in it....
The problem is legally vets cant dispense it with out seeing the animal. As they should technically be doing a heartworm test on the dog before prescribing the med. As giving an animal who has heartworms, a heartworm type med like revolution or sentinel will kill the dog.
So the ones who aren't selling it to you aren't being turds, they just don't want to risk their license.
However if you do phone around im sure one vet is bound to sell you something. Even post an ad on craigslist saying you're looking for a single tablet.
I used to always see people selling tubes of revolution on CL, im sure somebody would be bound to part with a single sentinel.