Hi, this is an important question. The thing is, online articles are usually hard copies as well, so even if you find them on Google Scholar or questia.com (2 very good resources) they also still exist as actual articles in scholarly journals. So, you can still cite them as journal articles as long as you spend the time to find the publication info.
Also, you really should ask your academic advisor about the details... perhaps your focus for the dissertation is particularly suited for online sources (i.e. maybe you are writing about E-Commerce)?
So, 1.) ask an adviser, and 2.) even when you read sources online you can still cite the original publication.
Kevin, EssayForum.com