Your dog is aggressive. I get it. More than half of my caseload as a professional dog trainer is to help rehabilitate aggressive dogs. Often it is overwhelming to know where to start or how to solve the problem.
Before you find a good positive reinforcement trainer, you need to face the facts. Your dog is aggressive and every time he or she reacts to another dog your job will be harder to solve this problem. The best way to treat dog aggression is to use desensitization which involves slowly introducing your dog to another dog at a distance that he is comfortable.
I routinely see people in my neighborhood and when I am working my my client's and their dogs that do not keep their dogs under control. My three dogs lie in our small yard which is on a busy street. I routinely see people walk by my yard with their reactive dogs that end up barking ferociously at my dogs. There have been numerous times that my dogs have almost been bitten through the fence. My fence is a sturdy wrought iron fence and does not have holes or other ways for my dogs leave my yard on their own.
My dogs also do not bark at other dogs normally. One in particular often lies with his nose close to the fence and watches people and dogs walking by. I would understand if someone is surprised by my dogs one time and then they take measures next time to keep their dogs away from the fence as they walk by. But it is mind boggling to me that I see the same people consistently walk by with their dogs inches from my fence, resulting in a ferocious outburst.
Besides my concern for my dog's safety, I feel bad for the reactive dog. The owner of this dog is not doing the dog any favors by allowing this repetitive behavior. I watch a dog reacting this way and I know that they are not as happy as they could be. A reactive dog snarling and lunging at another dog is not calm or happy. If I see it as often as I do, I guarantee that this happens more than what I am witnessing.
I believe that some people are in denial about their dog's behavior and, "maybe my dog will be calm with the next dog".
If your dog is aggressive, you need to be realistic about what is happening. Find a good trainer to help you and in the meantime, be sensitive to other people's dogs. It is not going to help your dog to "try" and meet other dogs if they routinely bark and get reactive. If your aggressive dog attacks another dog, the other dog could be emotionally traumatized as well as physically injured.
Let me know if you have questions, I can help.