Sorry you've been able to find medication of any kind.. what you have found can't help internal infections. :(

Are you sure he's biting it and it's not rot? When bettas bite fins I find there are no traces of the fins in the tank..

Is he still in epsom salts with 2 weekly water changes of 50% and 100%?

Hi there! Just wanted to update you on Prada. Turns out nobody sells any type of antibiotics in store anymore, other than BettaFix, Melafix, or PrimaFix, and I know those won't do any good. I've called pet store upon pet store and even some aquarium stores in the area and NOTHING! :(
Now I have to order online and wait for it to be shipped... Will it even work at this point, though? :/
Prada is still alive. Hasn't eaten much of anything, though. The poor thing... I know he's upset and stressed out because he's been biting his tail so much. I find flakes of it on the tank floor.
Thank you so much for your help, though! I'm doing everything in my power to try and save this little guy's life. But everybody says to not get "up in arms over a fish."