For body and trim parts there generally IS no after market. On things like you mention, you can go after market and save money - but watch the quality. 'Saving' $100 on an alternator by buying a Chinese knock off that fails or a rebuilt that konks out and has to be replaced twice (the rebuilders often screw them up and all you get back is a replacement part that may or may not be any better than the first one, no compensation for shop time wasted) and you'll at lkeast seriously consider going OEM on some stuff.
Unless the little bits (latches etc.) are common to enough cars that an after market version is warranted, you'll be out of luck on that stuff.
In any case, my point stands - until you know the factory price, you have no way of comparing what the after market is asking. It may be reasonable or it may be more expensive than the dealer. For dealers, I've found Crate Engines to be very responsive and much less expensive on shipping than others that advertise lowball prices and then slam you on shipping to make it up.