where can you get a wall mount for Mac Vesa

This forum only have users of Apple products, no Apple employees are here. You cannot buy Vesa mount separate, it has to be factory installed to the iMac, when ordering brand new. Check out this link: Choose an iMac with a Built-in VESA Mount Adapter

Maybe one of these will work:

http://imacmount.com/

https://www.tethertools.com/product/imac-vesa-vu-kit-connects-to-imac-base/

https://www.newertech.com/products/numount_vesa.php