Reality of obtaining tickets
If you haven't bought in advance, then buying in Aguas Calientes is the only option.
However, if you believe the hype you'll think this will involve a couple of days waiting and waiting in a queue at 4am.
The ticket situation changes day by day, but this is my experience from the 28th June, which is very different to what I expected.
I arrived at the ticket hall at 3:30pm and every ticket was available, but curcuit 2 tickets were only available for entrance after 2:30pm. This was a little late for.me, so instead I bought a 3B ticket for 7am. I then bought a second ticket for curcuit 1B for 11am.
To buy a second ticket, you have to return to the office at 9pm (start queueing at 8:30pm).
This gave us 4 hours in MP, seeing an amazing sunrise, the buildings, and the 'classic view'.
Also, even if you see curcuit 3, you will be very happy, so don't get too hung up about it if you miss curcuit 2.