When can I purchase an Apple iMac using Apple Card?

If you are in the U.S. you can do that today. If you don't already have an Apple Card you can apply on your iPhone as described in How to apply for Apple Card - Apple Support or you can apply while buying online as described in Apply for Apple Card online when you buy a product with monthly installments - Apple Support

We are all other users here and not Apple store employees, so we cannot give you an answer - but, I am puzzled: why can you not buy one now?

Apple.com is where I'd buy from!

Great, have fun with your new iMac when you get it!