Hi Alex & Woodie
Both are right.
You could download it or simply ask for one at Park's entrance and fill it out (as far as I remember that permit is given both in Amecameca and in Paso de Cortés).
Usually, what I do is:
*I directly go to Paso de Cortés
*Here, I have got to ask for one permit, fill it out and pay (less than 1USD by one day. If you are staying there 1 day and 1 night you will paying less than 2 USD)
And that's it.
I think it's quite practical having it before you get there however I haven't had any problem to access if I don't have it previously.
I hope it helps, and have a nice hike.