I have the same situation and question:
I do need to change the stand for a VESA mount and all I need (Apple refuses to help with this) to find out where I can get the needed parts and the VESA mount that goes with it. Any suggestions where to purchase it? Even if it a used one, has to be in good condition of course.
It's been a long time and I cannot return my iMac to exchange it for one that comes with the VESA mount. I even offered to pay for the shipping back and forth. The replacement and so on so that I can get the mount installed at the factory, they refused.
I told them that it was their fault that they do not inform people of such option. I'm very upset but very determined to get this mount for my unit.
Any help or suggestions on how to get this done will be appreciated.