So the tickets open 2 weeks (or 10 days?) in advance and get bought out in minutes through the official channel on WeChat. Both foreigners and locals use the same channel.
The tickets get sold out in minutes after they are released- this has been the case since about March. There have been a number of unscrupulous travel agencies that have been buying these up and then reselling them on the black market - some of them have been stopped recently.

There are a bunch of less scrupulous tourist agencies that will sell you the same ticket (200¥ vs 50¥ or something like that.), making it into a "package" with something useless. Check CTrip (or trip.com), Fliggy, QuNar (read as chu-naer). You'll need a local number to buy.

Mao-soleum - same issue.