Go here everyday around game time. They post tons of links for the streams. There's always a great youtube link to full 1080p stream, i use it all the time to watch out of market games. The mods remove the threads each day that's why it looks barren but around game time you'll see more stuff. They'll definitely have post season game too, it's pretty scummy of the mlb to charge subscribers even more to watch post season games.
https://www.reddit.com/r/MLBStreams/