Going to apple.com doesn't work. They will not allow you to find the plan much less sell it to you.
I found it in about 2 seconds at Apple.com.
There is a button at the bottom of the page to purchase. Of course if you didn't add Apple Care within a short time after the device was first new, your options for purchase may be limited.
You can find the cost of Apple Care on apple.com either as part of an "add to cart" transaction or on the support pages easily. It is available within 60 days of purchase. It is available as a stand alone purchase from an Apple store or by calling Apple Care @ 800-APL-CARE and cost does vary by model.