I've found the necessary answer on my question. But answer provide the name of sitecore support dll. How I can download necessary files if I know support dll name?
You can try searching for it here:
The dll will have a reference number associated with it Sitecore.Support.xxxxx.dll.
For example: https://kb.sitecore.net/SearchResults#term=442487
If you can't find it just raise a ticket with Sitecore and they will provide you with the dll.
Also recently released is this Github project containing source code for support fixes
Sitecore no longer provides support dll on https://github.com/SitecoreSupport. Having recently tried to access a support dll on github.
If you try and access a dll using a path you've been given previously you will receive a 404.
I asked a question relating to this on slack and Rob Earlam, Technical Evangelist @ Sitecore answered: “We don’t have Support DLL’s on GitHub anymore, if you need a specific fix then you’ll need to open a Support ticket yourself and make the request there.”