If you know a real vet and not a glorified anal-gland squeezer and nail trimmer, you can show them the original research from Dustin Dorton from ORA and they will likely prescribe you some. Explain that this particular bug needs 4x the dosage amount. Most vets are subsets of the profession and might turn you away, but real vets that do more than dogs and cats are usually understanding. There are many pet mets with Milbemycin Oxime and not just Interceptor. The ones with Prazi in them are fine to use - the prazi is not a super high dose in most cases.
All of that said, even at high dosages, it does not always work on black bugs. Don't expect many pods, starfish, shrimp, etc. to live at that dosage whereas they have a good chance at the original dosage amount.