Necessary, maybe not depending where you are at in your training, i.e., novice or expert. I would say that a trainer could push you past points you thought unattainable, when you start out, and they can give you tips on what to do for these types of comps that you might not know about unless you had experience. I would recommend one.
You already said most of the ways I would look for a personal figure comp. trainer except going to an actual show and asking around or seeing if there are advertisements for those types of trainers. Good luck and let us know what happens.... maybe a journal?