Portafilter pressure gauge
I built one with a small needle valve...
The brass fittings (including the needle valve) came from the local hardware store.
The silicone fluid damped, 0-160 psi pressure gauge came from McMaster-Carr: Part Number: 9780T215http://www.mcmaster.com/#9780t215/=5sipy6
And I did a little machine work on the brass fittings to make the assembly shorter so it would nicely fit.