There isn't a huge amount of profit wriggle room in a lot of camera gear; so if you are seeing items marketed as official; but priced way lower than major stores then chances are they are fake.
Some might be grey imports; which in themselves are not illegal, but can have restrictions on warranty cover (eg for nikon they have to go back to the country of origin if you want a warranty repair done); or might only have a store warranty cover rather than manufacturer.
There are also 3rd party options that won't be official, but should mount to the camera; some are great, some are good, some are bad so do due diligence and research to find the good choices.