One of the best ways is to search in Google Scholar (scholar.google.com) 

One of the best ways is to search in Google Scholar (scholar.google.com) 

Visit

ieeexplore.ieee.org

You can use Google scholar, Engineering village(https://www.engineeringvillage.com/search/quick.url), scopus search (https://www.scopus.com/home.uri)

Search on IEEE publications in http://ieeexplore.ieee.org/Xplore/home.jsp

sciencedirect publications in http://www.sciencedirect.com/

springer publications http://www.springer.com/gp/

and many other valid publishers like  wiley,  taylor and francis, ...

Hello Afsaneh

I can added also :

http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1098-2760

https://www.hindawi.com/

http://www.theiet.org/

http://www.jpier.org/PIER/

Best regards

Best free source is

 Google Scholar

http://scholar.google.com 

Also  Research gate

http://www.researchgate.net

For paid resources you can use following top 3

1)  IEEE publications in http://ieeexplore.ieee.org/Xplore/home.jsp

2) Sciencedirect publications in http://www.sciencedirect.com/

3) Springer publications http://www.springer.com/gp/

http://onlinelibrary.wiley.com

http://gen.lib.rus.ec/

You should be a litle mor specific. Source research depends what topic you are interested in.

IEEE publications is a good source.

For Power Trasmission and distribution systems IEEE Power Delivery,for Electrical Isolation IEEE Trans. on Electrical Isolation.

In spanish REDALCYT would be useful

Also IEE form England exists as a resource for several specific topics.

My experience is to look for some papers on the topic of intetrest,. Usually you can find some paper free download on internet. With one or two papers, with the references, you can track them on internet and  to browse dowm streams in order to get them.