Aftermarket and original equipment (OE) parts manufacturers are now typically one and the same. However, it is still sometimes the official OE brands that are the first to offer less commonly replaced parts for the newest vehicles.
RockAuto.com has had ACDelco (GM) and Motorcraft (Ford) parts for many years. You may not be aware that RockAuto now also carries the other American "Big Three" OE brand, Mopar. Need factory valve covers for a new Dodge Charger? RockAuto has those valve covers and other Mopar parts for late model Chryslers.
Some of the brands may not be as familiar as Mopar, but RockAuto has hard to find OE parts for other late model daily drivers too. For example, Auto 7 provides RockAuto with many complex OE parts for Hyundai and Kia cars.