What @janbb and @Earthbound_Misfit said.
When profs attend gown-worthy ceremonies at the university where they teach, that university provides the standard gown for the university that each obtained their PhD from. I’m told that very few profs carry their own gowns from post to post.
@ibstubro I wonder if the site you linked to is either meant to service universities in bulk, or trying to bilk students out of money for items that they should be borrowing from their soon-to-be alma mater. I’ve never seen anything like that before.