Well you can either select a SSD if you are buying from the apple store on the customisation pages or if you are wanting to do it at a later stage I recommend going to crucial's website located here: RAM Memory Upgrade: Dell, Mac, Apple, HP, Compaq. USB drives, flash cards, SSD at Crucial.com (make sure you select the right country from the top right), it will tell you which are compatible with your machine and give links for purchasing them.
Shop for your Apple, Mac, iPhone and other computer products on Amazon.
We are a participant in the Amazon Services LLC Associates Program, an affiliate program designed to provide a means for us to earn fees by linking to Amazon and affiliated sites.