how much photon energy would be required to produce a
proton-antiproton pair? Where could such a high energy photon come
from?

You need a photon energy that is greater than twice the proton mass.
Since the proton mass is about 1 GeV, you need at least 2 Gev. The
most likely source of such a photon on the earth would be from a
proton-antiproton collision in a particle accelerator or from a high
energy cosmic ray.