You can get the Dark series (that make you look like dark Link) from the Mon shop, but it's not there from the beginning.

What do you have to do to make him stock the Dark Series of equipment?

2 Answers

You need to defeat all of the divine beasts to unlock the Dark Hood, Dark Tunic and Dark Trousers at Fang and Bone at Kilton's shop, costing a total of 3,997 Mon.

Further reading: Kilton the Monster Parts Merchant on IGN Wiki.

1

All I did was defeat all of the DB's and Calamity Ganon, then I found out I could buy the Dark Link Set Using Mon.