Check the Find a Trainer
section. Most say what their specialty is and some do work with aggression problems. One near me (he's moved and I don't know where to) does temperament assessments at the OSPCA closest to him and does specialize in dogs with aggression problems. You could ask local OSPCA and other shelters and rescues near you if they could recommend someone.
Ask your Vet, boarding kennel, dog training friends too. Be careful, it's easy for anyone to say they are a dog trainer and hang out a shingle. No credentials or licensing is required. But at least if they belong to CAPPDT there is some reassure of integrity and knowledge. Or if they come recommended by such a person.
Probably the first thing any good trainer is going to want to know is the state of his health. Thyroid problems can manifest with aggression. There are other health problems that might do so too so your first action is to get a Vet check, explain your concerns and a full blood panel done.