where to purchase VESA mount adapter 923-05188

You won't be able to remove the stand from your iMac. To use a VESA mount the iMac must be ordered without a stand.

However, Amazon.com has many wall mount adaptors that can be used with an iMac with its stand. Like this one: VIVO Adapter VESA Mount Kit, Bracket Set for Apple 21.5 inch and 27 inch iMac, Late 2009 to 2020 Models, LED Display Computer, Stand-MACB

Not as pretty as a true VESA but it works nevertheless.

Yup, as mentioned you cannot add the VESA stand after the fact as the case is built for VESA or you buy an iMac that has a stand thus a slightly different case. This has been what Apple has done on the iMac line since 2012.