Travellers are usually found on Space Stations or Trading Outposts. Maybe not focus too much on finding one and do something else while keeping an eye out. Once you do find one, you might want to repeat the same traveller instead of finding new ones. There are a few topics on this subject:
As for the Specialist Terminals, once you have finished all their quests, you can remove them if you wish. If you rebuilt one, it will be back with the guy you hired. HG may of course decide to add additional missions for them in future updates, so rebuilding may be needed if that is the case.