Wasd keyboards sells individual keycaps. In their product page, ~~they list the dimensions as XxY. https://www.wasdkeyboards.com/custom-text-cherry-mx-keycaps.html I dont know if thats standard practice, and on this site only that particular tall size. Not sure about in the general sense. Maybe you can search for 1x1.5 caps and see what you can find.~~ Edit: nope, they dont even follow their own convention and just write 1xK where K is just rhe other dimension. Sad.
You could always find a parametrized 3d model of a keycap in a particular style, and manually create the model and order to have it printed.