It would be useful if you said why you want a different measure. For example, the answers you will get will depend on if your reason is due to your data not conforming to assumptions of the traditional Sobel test (and there are extensions for, for example, binary outcomes and mediators) or that you want something conceptually different.

conceptually different, to understand different methods and applied for research

For a conceptually different approach, see http://imai.princeton.edu/projects/mechanisms.html

Yes, you should use a bootstrapping approach.

this is a fine discussion of different mediation approaches:

Zhao, Xinshu, John G. Lynch, JR., and Qimei Chen (2010), “Reconsidering Baron and Kenny: Myths and Truths About Mediation Analysis,” Journal of Consumer Research, 37 (2), 197–206.

Kindly find a detail explanation of Sobel test, Baron and Kenney mediation procedures and Bootstrapping method.

Article Making Sense of Mediating Analysis: A Marketing Perspective