It's super easy to make yourself. Just grill a burger however way you like, slap the patty on some cooked white rice (I use jasmine since I always have a 20-lb sack of it), slap a fried egg on it (usually cooked over-easy) and then some gravy with onions. For the gravy, you can make your own, but I just use whatever I can find at my local grocery store and heat it up after I sautee onions in the pan.
Tastes just as good, if not better, than the ones you can get in Hawaii. It is a simple, cheap dish, after all. :o)
Good luck. I haven't had any success in tracking down a local style Hawaiian restaurant. You won't find this kind of fare in say Roy's, a place that uses the Hawaiian moniker quite liberally. After living in Hawaii for years, I've eagerly searched for good every day Hawaiian food, or at least the plate lunch variety. This style of food is about cheap food with good size portions for hard working people.
Then again, Loco Moco is rice, hamburger patties, brown gravy, and maybe some eggs; served with a side of macaroni salad.