The rear sight standard dovetail is, in my opinion, the weak design point on an otherwise very customizable gun.
That said, your best options are czcustom.com and cajungunworks.com
Alternatively, you can send your slide to czcustom and they will mill the dovetail to fit more widely acquirable dovetails like Novak style (look at the hiene sights on their top end custom guns). Last, I see Dave at CGW is designing a new rear sight. Not sure on timing of its availability.
Good luck!