That HP official black plastic front fan holder/fan kit for the Z400 is hard to find now, and expensive.
Rather, you can use a HP 4-wire PWM fan from eBay and add one in yourself.
Use this forum's search box near the upper left to search for "ghetto mod" and you'll see how I've done it in the past. I've used that for the earlier xw generation of workstations, and also the xw4600 which has a very similar case design to that of the Z400.
The 92x92x25 mm case fans work fine for this, and those often have a brownish red plastic motherboard header plug end.
