How to get it?

If you already have lean and its standard tools:

$ git clone git@github.com:leanprover-community/lean-perfectoid-spaces.git
$ cd lean-perfectoid-spaces
/lean-perfectoid-spaces$ leanpkg configure
/lean-perfectoid-spaces$ update-mathlib
/lean-perfectoid-spaces$ leanpkg build

Cut’n’paste-friendly version:

git clone https://github.com/leanprover-community/lean-perfectoid-spaces
cd lean-perfectoid-spaces
leanpkg configure
update-mathlib
leanpkg build

If you don’t already have Lean then you can follow instructions on this guide