Blank black PBT
with windows in OEM profile will be VERY hard to find.
Even just a good set of blank black PBT is hard to find. AFAIK the Ducky sets (OEM profile) and Ivan's GB (Cherry profile?) are the only places to get them.
If you don't mind the difference in texture, you could get the Banggood Blank Black POM 87 or 84 key set. The 84 key set has 3x single keys with windows and 1x caps lock with window. The 87 key has 2x singles and a caps lock. OEM profile.
May be worth getting just to try POM, maybe you will use the whole set on your board
Otherwise you'll have to buy a full Ducky set. That way, you've got some spare caps, too. Only option I can see for the correct blank black PBT with windows you want.