Where can I find a high quality copy of Turing's On Computable Numbers?
There is a very high resolution scan in the Turing Digital Archive, however it's presented via a flash player plugin which makes reading mildy awkward and doesn't seem to allow printing.

There are a handful of marginally nicer PDF copies than that Caltech one you link available online [1][2], however they all seem to have been drawn from the same photocopy at some point.