what do you mean by brand specific?
bearings are spec'd out like so many things, but they do not exist before an application creates those spec's. So when a manufacturer creates "something" that needs wheel bearings, they take into consideration all the engineering data and seek out a bearing/axle/drum(rotor) combination that will all work together. So, what the mfg of the "something" specifys a particular bearing, it is brand specific. But, it is a bearing class, not mfg specific-the bearing can be mfg by anyone as long as it meets the specifications placed on that particular bearing. Sometimes a single bearing MAY only be made by one mfg. Other times, demand causes more than one mfg to build the same bearing.
But, AS determines what bearing your outfit needs, so a contact with them is in order. that contact can be direct, or via a dealer, or via a manual, or perhaps a parts supplier application catalog.