Which scriptures say that Lord Kalki will take incarnation in the current (28th) kaliyuga?
No, we know for a fact that Kalki will be born in the 28th Kali Yuga of the Vaivasvata Manvantara. Here is what this chapter of the Matsya Purana says, in the course of describing the various incarnations of Vishnu in the Vaivasvata Manvantara:
At the close of Kaliyuga there will be the Kalki mainfestation, in the house of Vishnuyasha, and the sage, Parasharya Vyasa will be the officiating priest. This will be the tenth incarnation, and Yajnavalkya will make his appearance before this coming mainfestation. This incarnation will destroy all the wicked ones and the hypocrites; and, with a large army of the Brahmanas, He will kill Sudra Kings and drive away all hypocrisy. After killing the enemies of Brahmanas and other enemies, He will march on with His army in the 28th of the Kaliyuga, and then, after purifying the Sudras, will cross the ocean, where He will destroy the sinful mixed castes and, thus fulfilling His mission, he will educate his peoples.
It's also worth noting that, as I discuss in my answer here, Vishnu has taken Kalki-like incarnations at least once before, in one of the Kali Yugas of the Swayambhuva Manvantara.