Question

What are the sources which help us in reconstructing Jainism

Answer 1

Its still a work in progress. We currently only have secondary sources of information regarding their origins - for Jainism the Agama texts which was a reconstruction of the still missing Purvana texts that purportedly contained the original body of work. 

Buddhism can be traced back to its origins only from existing criticisms written by intellectuals and cult movements that was sparked during a time of great religious turmoil in the Indian continent.