Is it possible to buy these armours or is smithing the only way to get them?
There are two ways to get them outside of smithing - You could perform the conjuration ritual spell (req. Conjuration 90) from Phinnis at the College of Winterhold which rewards you with a sigil stone.
That stone is used to upgrade the Atronach Forge in the Midden underneath the college. That forge allows you to make either unenchanted Deadric armor and weapons or enchanted ones
The other way is only applicable if you have the Dragonborn DLC; open the black book Untold Legends and at the end choose the black market power. You can then summon a Dremora merchant that, if you have the right level, will sell Daedric.