There is the common one at 475W and the rare one at 600W.
With almost all HP parts there is both an Assembly P/N and a Spares P/N. Both are provided below, which will help in your search. There also are hacks that you will be able to find for converting a non-HP power supply to one that will work with the Z400. Personally, I would always search out the HP 600W one and pay the extra if I needed that extra wattage.
Z400 Power Supplies. Search under both part numbers, for both:
Delta 475W DPS-475CB-1 A (have seen Revs 02F, 03F) These are common to see on eBay...
SP P/N 480720-001 from Z400 Spare Parts web page (AS P/N = 468930-001)
Delta 600W DPS-650LB B (have seen Rev: 01) These are rare to see on eBay....
SP P/N 626409-001 from Z400 Spare Parts web page (AS P/N = 626322-001)