You're referring to someone to act an escrow service for you right if am correct, we don't use the term broker in this context here, there are two ways to get that done, you can either go through the service board under economy and go through many advertisement on services, there are reputable members you could check their profile and threads to see what and how they offer their escrow service, you can also check centralized exchanges they perform the service of an exchange, you can use atomic swap as well and the likes to exchange what you want.