You would need to contact Sales to order them from us, but you shouldn't have issue getting a replacement. Sales uses a database that I have no access to, so I wouldn't be able to provide any information from that side, just information from the tech support angle. Looks like the number you would want to use would be (877) 289-3355.
I'm showing 'PowerEdge R940 fan 3 low RPM' on the display.  And when I look on the back, the #3 fan is not spinning or is very slow, so I need to replace it.   So I went to [URL] to order the fan, but it doesn't give a part number.   How can I order the correct fan?  
PowerEdge R940 fan 3 low RPM