Welcome to the forums! Most gen3 distribution cartridges were attached to the GBA console and would look something like the picture I attached below. A general rule of thumb about distribution cartridges for Gen3 is that if they aren't connected to the console like this, they very well could be illegitimate.
I'm curious, do you have any distribution cartridges yourself already? Besides for private sellers, eBay has been the most likely place in the states to see anything related to distribution cartridges.