From the Super Mario Wiki
Sir Dominoes are only encountered in the area Frantic Factory, and even there, they are rarely encountered. A Sir Domino can be encountered in a large room filled with giant toy blocks. Two more can be encountered in the room where Chunky Kong fights the Toy Monster; several Sir Dominoes also compose the Toy Monster's legs. Another Sir Domino can be found in the room where Diddy Kong must solve a number puzzle.
A Sir Domino can be defeated in one hit.