The heavensward fantasia was part of the collector's edition in-game bonus items. If you did not originally purchase the collector's edition, you can use the digital upgrade service on the mogstation to receive all the collector's edition in-game items.

If you plan to purchase a fantasia, and you did not purchase the collector's edition initially, then you should probably go this route instead as you'll receive additional rewards on top of your fantasia. Also the collector's edition rewards are account wide, so if you make additional characters, those character will also receive the fantasia as well.