Most of the parts can be ordered off Digikey/Farnell/Mouser/RS. 

Magnetics can be ordered as samples from Coilcraft, or bought from them directly.

PMT and crystal can be bought off eBay.

Boards can be ordered for cheap from PCBWay, JLCPCB or OSHPark.

I've designed this project so as to use as few esoteric parts as possible.

Easy enough... A bit of SMD soldering, a few THT components, a PMT socket and you're set. The PMT socket should be mounted on the other side of the voltage divider (round PCB)

You should measure the current draw of the supply (which should be <50mA under no load) and the voltage at the output (which you can adjust from ~600-1100V using the trimmer).

Make sure nothing is getting hot.

Pay close attention to soldering the propper resistor in the propper place. If you switch resistors around, you could damage your PMT and, if you don't, you'll get worse performance.

Solder the socket on the other side of the PMT. Socket should come with the PMT.

If you use the round PCB, the voltage divider won't fit in the 3D printed housing. I've designed that enclosure for 28mm diameter PMT tubes with the voltage divider soldered on their socket.

I've made the PCB for a better PMT and for ease of maintenance.

Fit all components, drill 2 holes for 2 BNC connectors in the back of the detector. Solder 2 BNCs and wire the detector up. One BNC is the negative HV supply, the other is the signal out. DO NOT MIX THEM UP

Get a metal enclosure, drill holes for 2 connectors - 1 BNC and another one of your choice for 5V power.

Drill another 2 holes on the other side - one for the Enable Switch and one for an optional multiturn 5K potentiometer (do not populate footprint on PCB, solder wires and connect to panel-mount potentiometer.

Use plastic standoffs to isolate the PCB from the metal enclosure. Use an insulator to insulate the BNC from the housing. Alternatively, connect the shell of the BNC to the case and ground the case.

If you want to go all-out, use an SHV connector, such as this Radiall one. Warning, expensive parts.

PSU to detector, detector to 'scope. 

Turn them on and see what happens.

If you see negative pulses, varying in amplitude, you've done well :D 

Enjoy!