i just want to know that is it any site available where i found content related to my topic and make some changes and use in my site as a content.
you can take the free articles and publish them on your site but i think you will need to keep the Author name and source along with it then maybe you can use it
Those articles need to be kept as is. What you want is PLR or Private Label Rights bundles on your topic.
Do a search using your topic(s) and “Private Label Rights”
So, how much rewriting will you have to do to be safe? About 100%, but look how much time you saved.
Why not just use some articles from the various article directories, leaving the author box intact of course. That would give you the content you need for your site, and the Google duplicate content penalty is actually a myth so don’t worry about that.