Replace stand, need part number, 2015

Sadly, this is such a new system I don't think you'll find the stand available. My quick search online didn't show any available.

I would recommend you visit an Apple Store or an authorized Apple service center to see if you can get it covered under warranty or at least let them order the part for you.

Now the other shoe falls here...

This is a very big job! The complete system needs to be taken apart to get to the screws than hold the stand.

I must warn you you need some special tools to take the display off. If you don't do it correctly you can kill the display! Review the IFIXIT guide here: iMac Intel 27" Retina 5K Display Logic Board Replacement. Note the first 22 Steps is just getting the display off! None of the older systems require this much effort. Now jump down to Step 53. This is as far as you need to go to get the back side of the case to remove the screws that hold the stand.

If you let Apple do it you can get the bench fee (fixed labor cost) plus the cost of the part (and an extension of warranty). This is if you can't get them to repair it under warranty (do you have AppleCare ?)

Good Luck!