After a super quick Google search, I found a Halloween type mask for you:
amazon.co.uk/Fancy-Dress-Villian-Halloween-Costume/
I assume if you're wanting a decent replica, you'll have to contact one of the companies:
- Warner Bros. Pictures
- DC Entertainment
- Legendary Pictures
- Syncopy
I'm sure for the right price they'll be happy to make you one, though I'm sure if you can afford to fork out (probably thousands) on a mask, then you could just offer to buy the original.