27 inch iMac bought without computer stand but instead with a visa plate for computer arm. Now I am moving and cannot mount the computer arm. I need to buy a desktop computer stand for my iMac, which does not have one.

Replacing the factory stand would require finding the correct stand for that year model, plus complete dissembling the iMac.

A better option is to use an aftermarket visa desktop stand.

https://www.amazon.com/dp/B099MZCZ6F/ref=sspa_dk_detail_4?

https://www.amazon.com/dp/B0875SL7VL/ref=sspa_dk_detail_5?

If this is a more recent iMac which I think it is, then you will need to find some sort of stand that can attach to the VESA plate. When Apple sold these more recent iMacs with a VESA mount, the model with the stand and the one with the VESA Mount from the factory are completely different on the iMac housing. AFAIK there is no way to remove the stand of that model to install a VESA mount like was possible with previous models which had removable stands. So there is no OEM stand if the VESA Mount can even be removed.

It should be possible to find a stand with a VESA mount option.

they're sold separate, but don't think from apple

https://www.google.com/search?q=imac+stand&tbm=shop

You want one like this to color match your iMac:

https://www.amazon.com/dp/B00U8KSWB6/?coliid=I2ZIZAIIS4C9PI&colid=16WHQQ1HVMSYP&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it