Q. How can I get the article Managers and Leaders: Are They Different?

The article cannot be obtained through the library's Harvard Business Review in Business Source Complete.  

You can find it elsewhere, including

https://hbr.org/2004/01/managers-and-leaders-are-they-different

SJU users will find a reprint of the article in the following ebook (accessible with SJU login):

Champy, & Nohria, N. (1996). Fast forward : the best ideas on managing business change. Harvard Business School Press]. 

and reprinted in The Journal of Nursing Administration (JSTOR - SJU login required):

Zaleznik, A. (1981). Managers and Leaders: Are They Different? The Journal of Nursing Administration, 11(7), 25–31.