They exist but they're small potatoes. You have AAA-rated corps on the Corporate Council, then the extra-territorial AA-rated, then the "minor leagues" of A-rated corps, but there are plenty of businesses out there whcih aren't rated at A-status, like Fred's Pancakes House. He has a few franchises, operates out of several cities in the northeast, but a dozen eateries does not an A-rated corporation make.
Given a breakdown off the top of my head, I'd say wealth goes roughly like so:
AAA-rated - 60%
AA-rated - 20%
A-Rated - 12%
Unrated - 6%
Shadows - 2%
(Shadows economy being things like paying babysitters in cash, paying a friend who fixes your car, paying illegal immigrants off the books, and so on.)
That probably inflates the lower groups at the expense of teh upper, but for "pulled out of my hindquarters" numbers, they don't look terrible and explain the gobbling nature of the big guns: WIth 10 AA-rated, they each have about 6% of the world's business wealth, while each of the A-rated ones have around .024% each, making them essentially prey if they catch a big guy's eye.