Neosulf is neomycin sulphate used for infections and infestations. Try asking a friendly GP, vet, or chemist, as you would only need a very small amount. My Dr gets me Erythromycin when I need it (mind you, he only does it so he can tell the hospital his receptionist has algae :lol: )
If you need it for an external infection, ask female friends and relatives if they have any Pimafucort cream
Not sure what it would do for the fish though as it also has hydrocortisone and natamycin in it.