Is there any educational value into reading the original work of the authors who discovered certain theorems or concepts? I am interested in reading original work of some authors of theorems or concepts in mathematics because I believe that there is also an educational value to this and it might help me understand better those concepts because I can see where and how they came from from the author's minds.
For example I would be really curious into reading the work of Leibnitz and Newton regarding calculus, differentials, and integrals. 
Do you think it could help me better understand those concepts?
If yes, can anyone help me to find those original works?
 A: Maybe
I found people saying that it's useful, but no empirical evidence.
Example
In their paper Teaching and Learning Mathematics From Primary Historical Sources, Barnett et al. describe their method of introducing students to topics via historical texts.
Motivation
From the paper:

Thus, we are using primary sources not only to introduce mathematics via authentic motivation, but as challenging texts for students actively to “interpret” as a component of creating their own meaningful understanding of modern mathematics. Our tasks for students now therefore often incorporate a more active “read, reflect, respond” paradigm.

Usefulness
I could not, at first glance, find any empirical analysis of the effectiveness of this approach. They do get positive feedback from their students via a post-course questionnaire, though.
Access
I am not aware of any specific repository for historical mathematical texts, but I imagine they can be found like most historical texts (e.g. on archive).
See also

*

*One of the authors of the linked paper has a website on similar
courses.

*More on the pedagogical goals of those courses.

*For ideas of how to find literature without having institutional access,
see here and here.

*However, since you will be looking for historical texts that should be in the public domain, you can make use of the gutenberg project and similar projects.

*Honorary mention: A beautiful online version of The Elements of Euclid.

