Converting 3-pin brushless fan to 2-pin
Usually the 3rd pin is the &quot;tachometer&quot; output so that the fan's speed can be read by whatever is controlling it.
So in the case of a 2 pin 12V fan, you need only to connect the power leads properly and leave off the tach output since nothing in your PSU is expecting it.
As far as the connector, it's often simplest to cut the wires off the old fan and simply splice the wires onto the new fans leads.  Finding such connectors by themselves can be tricky.
Do not substitute a 5V fan for a 12V fan.  While it may run for a while, the fan will likely overspeed, overheat, and likely fail in a relatively short period of time.