While there are companies that assign ISBN to self-published books, this is not the correct use of ISBN. ISBNs are supposed to be assigned by the publisher. For a self-published book, the author is the publisher. So, someone else should not be assigning an ISBN to a self-published book. In some countries, there are resellers for ISBN, who can legitimately give you an ISBN that you own and hence can use for your self-published book. But there are no resellers in India. There is only one government agency, which is in charge of assigning ISBNs. So, the best thing to do is to apply to the agency and get one for yourself. And it's not difficult. It can be obtained from the ISBN agency for free with a simple application. Check out the application format and other details in this article on ISBN.