The NHL doesn't have an official source for starting goaltender announcements. There aren't any rules, aside from the immediate leadup to puck drop via lineup cards as to who will start.
That said, the evolution of the probable and confirmed starters on social media across the past 15 years has been impressive. Today, there exists an ecosystem in which the news oftentimes comes out well ahead of gametime. Some sites even archive this information with sources of the news.
That will be helpful as we wonder whether the announcement of a backup goaltender's appearance can move the lines.