Here is ZUN's blog where he announces things.
th17 trial was released at Reitaisai 16, which opened on May 5th 2019 at 10:30 a.m. JST
th17 full was released at Comiket 96 Touhou Day, which opened on August 12th 2019 at 10:00 a.m. JST
Where can I buy the game(s)?
Steam or DLsite! Some other great places to get the physical games include Amazon.com and J-LIST/JBOX. You may also want to checkout local stores and conventions.
Where can I discuss the new Touhou game(s)?
So many places that I had to make a place to list them all! That place is here:
This site should have X!
We (probably) agree! If you would like to add it please feel free to add a pull request on our Github! This website is 100% open source.