I have no ideas about brackets but i'm the first thing people will tell you is that it is a bad idea to install a tv above a fireplace. unless of course its not in use
Indeed. It is not a good idea to potentially expose a TV to smoke or heat.
You might want to consider another room for your TV, if possible. If not, try another place in the room, away from the fireplace. Of course, I know, you have your furniture oriented for the fireplace, and that it why you were thinking of putting the TV over it. Unless you have money to burn and don't mind buying a new TV frequently, rethink your plan.