Where can I purchase this online article?

That sounds like the title of a periodical. If that is the case your University library (or one close by) will have it as they keep current and past/bound periodicals. You can just make a copy of the article. If they do not have it your library's inter-library loan office can get it for you as a copy from a library that does have it, and it will not cost you anything. If the library for some, ridiculous reason can not get a hold of it (which never happens), or it is not a periodical, the official website of the publication will sell it to you, but you will need to find the official one, not a hosting site.